I have 300GE, short, 1991 and a G55 AMG, 1998, which is earlier version of the G500 you talk about (my car has M113 engine without compressor, 5.5l, AMG modified).
I am not sure what is your experience so far with G-class.
In essence: the choice depends on your personal preferences and on what you need the car for. For off-road - short version, diesel. For every day - long version gas. For long trips - long version diesel (in G500 you will make max 400km on one tank). It will always be noisy at speed as it has aerodynamics of a 3-door closet (which is what i like about it). At 55mph is not noisy yet. at 80-90mph it will be. G-class diesel will always be a bit rough. I have recently been driving a new G350 - a bit rough. I have an ML with the same engine - it runs like a silk. But maybe you like it rough. The only nice thing about diesel is the high torque at low revs and range at moderate speeds - 1000km if you are careful.
I do not know the company you mentioned. But I know G-class is easy to swap engines and gear boxes. I saw many well done cars in the dodgy garages of G-class mechanics (special species in my country). But if you like G-class diesel, why don't you look for an original one? Please note that often engines for G-class differ from the same engines used for passenger cars. This may be, however, important while off-road driving only (I am not sure) - electric components installed higher, different oil pan for steep climbs and descents etc. Not sure what version is installed in the car you are talking about. Gear boxes may differ too, I am not sure.
When driving/looking at it - pay attention to vibration (it has 3 shafts, 6 joints, 3 differentials, one inside the transfer case - they can get messy). Look at overall drive train condition - it is a lot of parts, all expensive. Pay ttention to noises from transfer case and from differentials. Make sure the elctrohydraulics locking the differentials work - difficult to fix and they get stuck if people do not use them from time to time. You may not be able to lock a differrential also if the axel is bent after som ecollision. To lock a differential you may need to drive for some meters and turn the steering wheel - the locking mechanism may require the wheels turning at different speeds for a moment to engage. Press the buttons in proper order (center, rear, front), otherwise they will not engage. Pay attention if steering box is not leaking - not everybody can fix it and even the second hand one is expensive. Pay attention if sliding roof works - very hard to get good used brackets and new ones are expensive (do not ask me how I know it, e.g. for W124 you will have hundreds of them, for W463 they are rare, they look alike, but they are different). Pay attention if low range engages in the 2007 car - same reason for being stuck as differential locks. For 1990 car it does not matter.
Wheel rims that fit G-class are G-class rims. Others do not fit. Some say Sprinter rims also fit, I am not sure.
Make sure the geometry is ok - the car has a frame, it helps if it is straight. Make sure the condition of the frame is ok and VIN is clear and visible (right wheel arch, stamped on the frame). Rust appears on the front floors (raise the carpets), rocker panels, windshield frame area (clogged drains) and in the inner spaces above the rear plastic wheel housing. Sometimes you see it coming out around rear lamps. Rear panel door gets rusty too. Make sure this door sits properly, otherwise it makes lots of noise.
These are top of mind thoughts. Sorry if you know all this.
