I have owned a few Range Rover classics, Disco I and Disco IIs, all NAS (North American Spec). I would get the Disco I over classic any day, especially the 96 and on with GEMS engine.
The frame and chassis of SWB (Short wheelbase) Classic is very similar to that of Disco I. In fact, quite a few parts are swappable between the two. Being a later and improved model, Disco I has the following advantages over Classic:
1. Not as rust prone, none of that aluminum body parts bolted on to steel ones silliness.
2. Electrical system is better. Things like power windows, mirrors, sunroofs are a constant PITA on Classic. While Disco is not perfect, the electrical system is clearly an improvement.
3. Some really old Classics supposedly had 3-speed Chrysler transmission. The newer ones had ZF 4HP22 which is better. Discos only come with ZF trans.
Here's a rundown on their engines:
1. 87-88 Classic - 3.5l, too wimpy for the heavy rig, distributor driven POS oil pump. Upside: block walls between cylinders are thicker, slipped liners are not as common as with later engines. Ignition is Lucas somewhat problematic "high energy", the amplifier when fitted to the DLM8 distributor is an unreliable POS, needs to be relocated to the fender to work reliably. Upside: fuel injection is rather simple 13CU, easy to work on
2. 89-92 (93?) Classic - 3.9, more power (still not adequate, but an improvement over 3.5), thinner block walls (problems with slipped liners start here and persist all the way through 2004). Oil pump - same as 3.5. Ignition - same as above. Fuel injection - 14CUX, simple easy to work on.
3. (93?) 94-95 "Interim" 3.9 fitted to both late classic and early (94-95) Disco I. These engines have better oil pump (crank driven as opposed to distributor driver). One serpentine belt in the front instead of multiple V-belts. Otherwise the same as earlier 3.9
4. 93-95 "4.2" engine - fitted to LWB (Long wheelbase Classic), this would have been an adequate engine for SWB. Too wimpy for heavy LWB classic. Can be fitted to pre-96 Disco I and SWB classic as an upgrade.
All pre-96 engines don't have crossbolted main caps. This means that the main caps are subject to fretting, which allows the crankshaft to flex too much, which in turn causes it to break. All these engines need to be upgraded to ARP main studs instead of bolts to avoid catastrophic engine failure.
5. 96-99 "4.0" motor (not fitted to Classic, only Disco I) is better in many ways: crossbolted main caps, bigger diameter crankshaft, crank driven oil pump, GEMS distributorless engine management system. The 4.6 motor from Range Rover HSE (P38) is a direct bolt-on. The swap is straightforward and gives you adequate power and torque.
6. 99-04 "4.0" motors (As fitted to Disco II), same as above but fitted with Bosch Motronic instead of GEMS. Good for 20 or so more ponies than GEMS, otherwise the same.
All Land Rover aluminum v8 motors made after 1988 do not tolerate overheating at all. Thin walls crack, the quality of casting is **** anyway, plenty of porosities, the cast iron liners slip and the engine is ruined.
Overall, the V8 motor is the weakest most troublesome part of the Land Rover. Short of rusted bodies and bent frames everything else can be dealt with easily. The upside is that a Disco with a dead motor is worth nothing, I buy them for $500-$1500. They are disposable trucks. If you go offroading and wreck one, you can always get a replacement Disco with a dead motor from CL very cheap. If you I have the engine swap figured out, It takes one week-end to do.