Hi and Welcome to FOG!
Right, as you have mentioned the 2.4 Im only going to talk about the LWB models here and the A series.
The move to coil rear suspension was in 1995, so any N reg model onwards. They are a bit more comfortable and handle slightly better on road.
Many parts are shared with the Isuzu Trooper, chassis, some gearboxes, brakes, suspension so parts are readily available.
Tax is cheap on all A series as pre 2001 so not based on emissions.
95' on models come with a few extras which may not be on earlier models, electric windows, central locking, electric heated mirrors etc.
Its a good long distance car in any engine form, I used to regularly drive from Ireland to london and latterly london to Wales and still feel comfortable after a two or three hundred mile journey.
Starting with the 2.4 (91' to 94'). Its a solid reliable engine (taken from the Opel Manta with and fitted with an 8 valve head) producing 123BHP.
One member had one which had done a bit over 200,000 miles when he sold it and it recently came up for sale again still in good working order.
Fuel consumption isnt great, averaging 25mpg give or take and depending on driving style.
The 2.3TD engine (91' to 94') is taken from the Bedford CF van and again is a solid reliable workhorse producing 99BHP.
Average fuel consumption is about 30mpg, again depending on driving style.
Both of the above are getting on a bit now and many parts for the 2.3TD are getting scarce or impossible to find. The 2.4 shares many ancilliaries with other vauxhall/opel engines but catastrophic failures of both are
not common. Half decent examples should be approx £500 with some tax and mot.
Moving onto coil sprung models you have a few more choices.
95' to 98' 2.2 petrol. If you get a good one they will last but there have been so many reports of failures including big end failure and they have been known to throw a conrod through the side of the engine block (most likely cause for this is poor servicing) and head gaskets are prone to going. however, if you find one with 100,000 miles or more on the clock its probably going to be a good one and will last.
Produces 135BHP but you wont know it as its still a heavy car. Torquey enough engine but like all the frontera's its a plodder.
Average about 25mpg to 30mpg although I have gotten mine to do 37ish when I was careful
once.
Mine failed with approximately 70,000 on the clock and cost an arm and a leg to get a replacement engine. In all honesty, I couldnt recommend a 2.2 to anyone even though I am perfectly happy with mine at present (6 years of relatively trouble free driving since the engine replacement).
Good used examples available from £500 to about £1000.
95' to 96' 2.8TDi. Considered the most reliable of all the engines if not a bit agricultural. No clever electronics to go wrong. Fuel in one end and power and torque in the middle and clouds of diesel fumes out the back

Averages about 30mpg by all accounts if not more when driven carefully.
Produces 111BHP.
Expect to pay up to around £1000 for a really good one with tax and test.
96' to 98' 2.5TDS. The most powerful of the A series. This engine is fully fly by wire and the electronics can be a bit iffy sometimes. Some parts are hard to source and No1 injector failure is not uncommon and costs over £100 to replace IIRC, other tricky thing to get hold of is the viscous fan coupling if it fails but a member on FOG is working on a suitable and readily available replacement.
Complex electronics put many people off.
Produces 113BHP and puts my 2.2 to shame.
Averages around 30+MPG.
Price, Hard to say as they arent the most common but probably somewhere around 2.8TDi money.