2.5L was used through 2000 in the XJ and has the reputation of being as reliable as the 4.0L-- pretty much the same design minus a couple cylinders.
I've managed high 24mpg with my li'l 256000 mile grenade, but that's at 55mph on the Eastern shore (Delaware, Maryland, and Virginia) at sea level with a total change in elevation through 200+ miles of about 150 feet.
around town with a 2wd, 2.5L, 5spd I usually see about 19.5mpg-- that's 100% "hurry up and stop" commuting into and out of the largest Navy base in the world.
with 4.10:1 gears, it's got enough zip on 235/75's to keep up with everyone, but it seems to run out of guts on the interstate somewhere around 70mph-- she'll do more, but the economy drops quickly at that point.
If you're buying one to wheel, I'd steer away from the 2.5L just for the ease of finding replacement parts when you break it (AX-5 isn't as stout as the AX-15, etc) but as a D.D. it wouldn't be a deal breaker IMHO.