You need to know about the different models. There are the late LSEs (LWB to the US), which have the obvious advantages of more load space or more leg room in the back (the front seats do not slide further back, so if you're too tall for driving a standard 100" RRC, you'll still be too tall for a 108" LSE unless you modify it), but some parts are hard to find for these rarer vehicles, especiallt rear trim,rear door components/panels and rear prop shafts.
You also need to know about the two door and four door models, knowing how to spot the early, non-factory four door conversions like the Montiverdi - they'd be a nightmare to get parts for.
Rust affects the sills and body rear cross member, as mentioned, though the chassis is normally very good. The front inner wings, head light mounts, rear wheel arches (just inside the rear doors) and boot floor are all bad rust spots. These can all be replaced with Discovery I parts, which seem more readily available, though the corrugated boot floor panel's dimensions are different (normally replaced with two shortened DI panels stitched side by side or with a flat 3mm aluminium panel).
The tail gates are also rust-prone. Aluminium upper tail gates are OK, but they do flex when closing them one-handed, so shutting them can be more difficult if the seal is new. The upper gates are also prone to popping open if the latches are worn or maladjusted.
Engines are important. the V8 came in 3.5 (carb), 3.9, 4.0 and 4.2l versions. The 3.9 and 4.2 are prone to porous blocks, while all V8s suffer worn cam shafts if not maintained scrupulously. they are very sensitive to water or damp in the ignition system. The first factory fitted diesels were the Italian built VM units, first the 2.4 and then the 2.5. I don't know much about them other than that each cylinder has its own independent cylinder head, and that these were prone to problems. I think the 2.5 was an improvement, but I understand that the engines were all costly and unreliable, though very refined for a diesel of the time. The Tdis are relatively bullet-proof, though the 300 has a thinner head than the 200 which can distort if severely overheated. The 300 heads are still available new, but 200s are not. The 300 also has a mild reputation for head gasket failure behind the no.4 piston, blowing out the end of the block, and for the "P gasket" behind the water pump leaking (this requires removal of the whole timing case to replace). the 300 is perhaps a little less robust than the 200, but is much more refined and has a superior crank driven oil pump - I'd go for the 300 every time. the 300's timing belt issue was limited to the very early engines and was rectified under recall. Do not be concerned about this issue - any engine still running can't have been affected by it.
There are several different transmissions. ZF autos are very strong and reliable, but only if given frequent oil changes and filter replacement. many skimp on the filters or don't even know about them (the sump has to be removed to replace them), so failure of the hydraulic selection system is not uncommon. LT77s and R380s are good, but the LT77 (5 gears, reverse forward and left) is prone to main shaft spline wear because the LT230 transfer box input gears weren't cross-drilled. R380s are an improved LT77 (reverse is behind 5th) and doesn't suffer the main shaft problem, though both still wear their 2nd gear baulk rings after about 80,000 miles. The earlier four-speed manual LT85 boxes are tough but a little agricultural. Only the last of the Classics had the Borg Warner chain driven transfer box, which is quiet and strong and has a viscous coupling controlled centre diff. The other ZF and LT77 vehicles have LT230 transfer boxes identical to the Discovery and the same except for gearing as the Defender. Generally robust, watch for jammed high/low and diff-lock selectors.
The axles are standard Rover 2-pin diffs, most with 10 splines but later (early 90's onwards) 24 spline. The later 300 Tdi axles have stronger shafts and CV joints.
Brakes can be simple dual circuit plain discs, but later models have vented front discs, ABS/ETC and power assistance. That late unit is very effective and pretty reliable, but could be expensive to replace if faulty. The ABS works on all four wheels (the warning light should clear on reaching approx 5mph), while the same electronics use an alternative logic and the power booster to apply Electronic Traction control to the rear axle only. That light should extinguish shortly after starting the engine without needing to move off. Brake lines tend to suffer corrosion, especially if used on salted/gritted roads or near the sea.
Steering is basically the same on all models with a "4-bolt" (the number of bolts on the top cover) Adwest PAS steering box. They are prone to output shaft seal or top cover leaks as they age, but can be easily sourced from Discoverys and Defenders. Leaks can render them unserviceable, but they will normally function properly if the reservoir is kept topped up. If PAS failure does occur, the vehicle is still steerable manually.
Head linings are hard to get, but are prone to sagging on all models as the glue holding the fabric to the fibre backing fails. A trim specialist will be able to repair them in most cases.
Wiper units can become noisy as water enters the spindle tube and corrodes the shaft. Greasing may help a tired unit, but if it's already making noise, it will eventually expire.
Soft dash (late '94 and all 95 models) have SRS bags on the steering wheel and on the passenger dash. These are no longer available new, but the steering wheel is the same as used on the late Discovery I, so a replacement can be sourced there. The DI used a grab rail on the passenger side in the RRC's second SRS bag location, but it is possible a DII unit may fit 9though the colour would not match).
Useful upgrades for an expedition vehicle would be 4-pin diffs, stronger shafts and drive flanges, uprated suspension (UK police spec rear springs are great, along with DeCarbon dampers) and a storage system on the load space. Uprated suspension also gets around the problem of failed Boge struts (a self-powered hydraulic ram between the chassis and rear suspension A-frame that self-levels the vehicle regardless of load), which are damned expensive to replace and fail quickly.
Aircon would be useful on Vogue trim specs, but the Vogue SE (which has aircon as standard) suffers from leather seats (impractical as they get scratched and crack and also have troublesome electronic adjustment) and leaky sunroofs (nearly always due to blocked drain tubes which run inside of the head lining and door pillars). The SE and LSE also have Electronic Air Suspension. These were essentially test mules for the system to be introduced for the P38 Range Rover (second generation), and were even more unreliable than the much-maligned P38 system, with compressors burning out, valve blocks jamming, airbags perishing or puncturing, air lines leaking and height sensors being the most unreliable component of all. If to be used for expedition work, an EAS equipped vehicle would benefit from a coil spring retro-fit kit from the likes of Atlantic British, which includes the electronic shunt to fool the EAS' ECU into thinking the system is fitted and functioning so that it doesn't put other ECUs into a "limp home" mode.
Other useful mods for expedition or off road vehicles include removal of the front spoiler, diff guards, heavy duty steering rods, a steering guard (all the same as DI or defender), cross-drilled and spiralled brake discs (like EBC's) and a Southdown fuel tank guard with removable hitches and recovery points, (which attach directly to the tank guard when needed but don't affect departure angle like the standard hitch when not required).
