I am going to try and be as honest and unbiased as possible here
Trooper 3.1:
Good
After you have ripped the EGR's off and given it a bloody good service will return you up to 30mpg.
If an LWB could well carry more to to the larger boxy shape.
Parts are dirt cheap and avaiable from both Vauxhall and Isuzu if needed.
All Troopers and Montereys are the same. Made in the same factory and everything in Tokyo.
The "Import" Troopers that were oringinally sold in Japan and then imported over here tend to be in far better condition as they haven't been subjected to the salty roads we have.
Post 1995 Troopers had slightly more power.
No dodgy electronics, ECU's and the like ... just straight forward 12vdc.
You can slap a set of 31's on Frontera series A alloys and they wont rub even without lifting.
Lotus Editions have uprated front torsion bars, tougher leaf's and harder shocks i.e. they don't wallow as much.
Bad
Front/rear cross members and body mounts are the first place they rot, and have a tendancy to do so, but a weekend of wire brushing, rust preventative and a thick coat of hammerite or waxoly will sort it out. Many a Mk.II and Mk.III Trooper has found itself in the scrapper because of this.
Post 1995 Auto gearbox Import models have an electic fuel pump that is a pain when it goes wrong. All others use the tried and tested mechanical Bosch VE pump.
Pre 1995 Troopers had slightly less power.
The later SOTF models suffer the same issuse the Frontera SOTF system (perished vacuum pipes etc) Not a big issue, just annoying.
As for the 2.2 I will leave you to look on the forum. This is just from what I have learnt on the Trooper forums. Personally I wouldn't go for any Frontera unless it were a 2.3, 2.8, or 3.2 V6. It seems all the others have some fundamental flaw with them in one way or another i.e. 2.2 petrol blows headgaskets and manifolds with each season, the 2.5 is it which is fine but knowing my luck I would end up having to spend £1000 on a new fuel pump etc etc.
As said Just my opinion, and trying to be as honest as possible.