From my experience, it's not the Engines in the Gen I X's you need to worry about. ALthough the SC'd V-6 get's terrible milage for the limited power it puts out, it's the chasis strength that leaves alot to be desired. The motor is fairly stout and should, like a Toyota get you to 200K with little to no troubles. Those older X's and Fronty's are noticably "noodly" and have tons of frame flex that leads to them making a hell of a racket as they age. One friend with a 1st Gen S/C'D X is SO sick of all the squeeks (and the inscessent SC Whine... It's really pretty loud if you're not used to an SC) that he's going to great lengths to get himself into a Gen II.
I'd say that if you run close to stock tires you'll see nearly identical milage to stock, but going up in size will, as always lead to more rototng mass adn unsprung weight that never helps with vehicle dynamics (it only really helps with off-road performance). That said, why not go bigger and just re-gear??? THat's what I'll be doing to get back to a close to stck final drive ratio.
Nissan did ALOT to make the new Gen Xterra's ALOT more stout (excepft for the front Diff, can't explain that one for ya...). I have a few friends who have one gen or the ohter and they all like the 2nd gen the best. Like I said the S/C'd motor in the 1st Gen is a fuel drainer and really doesn't make that much power considering you could throw a Titan V8 in there and get better milage AND more power. The VQ motor in the New Fronty's and X's however is loved by all that have them and I think makes better power with a more linear power curve compared to the old S/C'd motor. Plus as I mentioned the new trucks are just made better, and IMO LOOK alot better (but hey that's all subjective so who cares right).
I feel like Nissan Tested the waters with the original X (and realized they had a hit) then improved on it in every way possible (outside of an SFA and that alu. front diff) on the next go-round. Hopefully you Nissan folks will get another version that's still a real truck instead of what's happening to the Patrol, Pathy and now the Grand Cherokee (as in IRS uni-body garbage)
Cheers and Happy hunting
Dave