Suburbans are very fun and practical vehicles in my humble opinion. They do provide a ton of room (a good deal more then Yukons and Tahoes as you have mentioned) and at not a whole lot of trade off. I believe the guy when he says he averages high teens for fuel economy, that is a nice benefit of diesels. But the gas 350s are capable of those numbers too if you get one from 92 on up, the 350s were reworked a little bit and their fuel economy was dramatically improved because of it. I have a friend with a 93 burban with the 350, has 285k miles and gets 18 mpg with it pretty regularly. When I am nice to my Burban I get about 12-14 on the highway as long as their fairly flat, but mines a pretty unsophisticated haha, but I do think that creates less headaches down the road, personally.
Lets see, I would ask to see if the turbo is working and if it is if it has been replaced? I know a few people who have had turbo issues with those 6.5Ls, which to me i wouldnt mind if i had the cash, cuz id just replace it with a larger and more efficent Garret turbo anyway, but if your not looking for that sort of expense then youl be especially curious about that. Make sure he let the turbo wind all the way down before getting back on the gas, alot of guys new to the diesel game dont do that and ruin their turbos pretty quickly becuase they rev it the way they would a gas, without letting the thing settle down completely again. Diesels are pretty good for many miles though, so if its been well taken care of then I wouldnt have too much of an issue buying or trading for one with that kind of miles on the clock. Ask him how much it has towed, if at all. Some people bought the 6.5L simply for the constant 17-18 mpg and never did an ounce of towing with them. Ask too about transmission maintenance. how often was it flushed? was it ever replaced? any shifting issues etc? does it get hot etc etc?
All in all, the 6.5L is a decent engine, but they are not huge powerhouses of engines. tuning the turbo to pump in more PSI and new injectors, 4 or 5 inch exhaust etc, wont gain you huge power. about the best 6.5L runs about 330HP to 400 tops, and after you reach that, your pushing the limits of the engine. But for a good old reliable workhorse, theres not many made better. But diesels are a bit more costly to own then a gas engine. They have many expensive parts, that can go out seemingly at the same time, and your wallet will be crying out in agony to pay for it. plus regular things like oil changes typically range between 80-100 bucks, depending on the filters and oil used and if you do it yourself or have it done by a shop. I know a 7.3L is about 120 bucks to change the oil and filter on, but i think the 6.5L would be a bit less expensive since it uses a little less oil haha.
Otherwise without lifting the truck you can fit on 33 inch tires pretty easy, but if you want to put on 35s or larger youd need like a 4 inch lift and maybe do a bit of cutting too, I have heard of people putting on 35s without the lift, but they had to cut away alot of truck in the process. If your interested in maintaining decent fuel economy and having a decent on road ride and still be able to get through almost anything off road, I would say look no further then 33s. Heck im running 31s on mine right now until i save up more for 33s, but I have yet to get stuck. Also, you would be surprised what these big ol beasts can get through. I have had a few times out when I thought it was too large to pass, and it made it just fine, I think a lot of people psych themselves out because of its size but never really test their theory, they just say "no, its too big." and call that good. lucky for me mines rusty and beat enough and cheap enough too that I have been able to be a bit more reckless with it then others, and they are very capable beasts. Be aware though that since they have such a long wheel base, they do bounce and jiggle a lot more then say a small Jeep would haha, but once you get used to the bouncing, it becomes pretty fun pretty fast, just make sure you dont have anything too valuable flying around in the back haha.
I'm working on mapping out a storage sleeping platform for mine, but once i go through it I probably wont be able to use my third row seat again, but I have slept in mine without that a lot with an air mattress etc, and man, these things are enormous! I cant stress how awesome they are for personal or family expo/fishing/camping use.
Both the 6.5L and the 5.7L are great, solid platforms so it really comes down to personal prefrence...and wallet for repairs. Having a diesel is always an expensive proposition. Repairs are almost never cheap, but luckily on 90s suburbans their a bit cheaper then on more modern ones, but cheap in the diesel world is a relative term haha. Personally if I were relying on something for all year use, cold starts and cheap repair bills, I would opt for a 100k mile 350. the 5.7L is one of the all time greatest engines ever built, mine has 150k on it and I know when my Lincoln is giving me problems, I can always rely on my ol Burban haha. But after 95 (and Im sure you know this from your own research, but just a refresher) they changed the cast material for the blocks to a cheaper metal and replaced the heads with more efficent Vortec heads. yeah the block isnt as good of material, but its still a good engine regardless, you just cant romp on them like the older ones really, but most dont do that to Suburbans anyway.
The gas job will aways start easier in the winter, even if you forget to plug in the block heater. I do think the reason my friend gets exceptional mileage is because he never ever gets into the throttle, but if your like me youl step into it at least a little to get up to speed quicker, and if thats the case then youl get minimum 14 and on longer drives up to the 18 mark. Plus 9 times out of 10 you wont have to throw it into 4 wheel drive since their so heavy and monstrous they can get through a ton in 2 wheel drive.
Plus the 350 will be able to tow most anything you need to tow, even a 25' RV if you wanted, just be sure to hook yourself up with the correct hitch for that kind of towing if you plan on that, so it wont sag quite as bad and ruin your suspension.
I know quite a bit about the 6.5L but in all honestly I am not too familiar with the 6.2L unfortunately. I know the early ones were basically a reworked 350 and were prone to excessive overheating and oil leaks and loss of pressure etc, but Im reasonably sure mid to late 80s ones had most if not all of those issues resolved, but I still am a little leary of them, in all honesty. The 6.5L I have always known to be a good strong reliable platform, but I could never trust the 6,2L the same way, you know? But for me to say that really is not fair to the engine if its problems were resolved. Heck Fords 6.0L had numorous issues, but with the right parts those can be resolved and it turns into one heck of an engine, and really its reputation is as bad if not worse then the 6.2L's...wish i could help more on that front, but I'm just not as well acquainted with them as I should be. All i can really tell you is stay away from the early ones, for sure.
