Need I answer this question

Trooper 3.1 obviously. Not as economic as the 2.8 but just as bombproof. Watch for post 1995 import autos as they have the electronic bosch VE fuel pump. Its reliable enough just a pain to replace IF it goes wrong. If you find a Lotus edition at a good price snap it up. They have far better suspension than the others. Lotus handling kit is exactly what it says. Much tighter when cornering and far more steady off road. The Trooper is quite a bit bigger than the Frontera mind so you will be sitting higher and the back is boxier so more load space in the back. I have seen some LWB Mk.I's floating around for sale at the moment and while a bit more basic inside they could have the good old tried and tested 2.8 TD under the hood (the SWB's have the 2.6 petrol) which will return low to mid 30's mpg wise (I got 37 out of mine a couple of months ago after 2000 miles of granny driving around town and bypass driving)
Earlier Discos, as said, are rot boxes ... and they don't dump the 2.5 diesel for the Isuzu 2.8 for nothing. Its a naff engine that cannot really handle the weight and load of the Disco. Yaki has poited out the rust areas and he is true to his word. Saying that a clean and tidy one will be loverly to drive on road for distances as the suspension is really soft ... some would say too soft for off roading as they do have a tendency to roll around like a weeble.
I love the Fourtraks. There small, light agile and basic ... with a massive heart under the hood that will runn all day as it has othing weight wise to drive. They are tough old things as well, but could do with a suspension lift as Yaki says.
Yota 2.4 have weak engines. They are also quite underpowered. My mate in his 2.4 pickup struggles to hit 70 in his rolling on 31's. Also the gearbox crossmember sits about 3-4" lower than the chassis. First target to get clobbered. Some Toyota parts are expensive to replace as well.
Another one to add to the list is the Isuzu Bighorn

.
.
.
.
.
.
.
.
ok thats just the Isuzu Trooper when it was sold in the home country Japan ... but I do mean something when I say that as the Japanese Domestic Market (JDM) machines have tighter suspension and usually more toys inside. Another bonus is they are less likely to have rotton front/rear crossmembers and body mounts.