The main reason G2's & G3's are cheap is because as soon as the owner is able, they buy a G4!!
It all depends on the aircraft you buy. There are some great bargains out there to be had, just ensure that you buy a good one.
Fuel. We work out that we burn approx 5,500 lbs in the first hour, then 4,000lbs the next hour, then 3,500 lbs the next hour, then 3,000 lbs there after, so a 6 hour flight would average about 3,500 lbs an hour.
Noise. We aren't allowed into certain airports or we have to apply in advance for an exemption. 99.99% of the airports we use, we have no problems.
The avionics is to be as expected from a 1970's design, but most have been updated to RVSM. G2 avionics are DC & mainly analogue, and later G3's have digital instruments & AC. When you are the owner, sitting in your big seat in the cabin, you don't really care what avionics you have in the cockpit!
Maintenance is what you make of it. If you have your own man in a van or a sympathetic organisation, it isn't a problem. If you use Gulfstream aerospace then they will find everything & it will leave the factory in a 'as new condition'. There is a very good maintenance organisation in South Florida. If you bought a G2 or G3 with a Gulfstream pre-buy check then you are getting a thoroughly well checked aircraft at a good price! We go to Gulfstream and over the last 'X' years each scheduled 6 monthly check costs between $250k-$400.
Parts. There are quite a few scrapped aircraft out there so 2nd hand parts aren't that hard to come by.
Reliability. We've never had a problem. We only have problems when we are at the maintenance organisation.
Prestige. Most customers (& most people) can't tell the difference between a G2, G3, G4, G5, G350, G450, G550!!
So if you had a G2SP with a nice paint job & a nice interior, the customers would be pretty happy. Most seat about 12 (some seat 19!) with about 6'2" head height
There are a lot of very experienced crew in USA as that is where most G2 & G3's are based. Not too many crews in Europe, Africa & M.E.