Welcome to LOC B)
Yes, the exhaust is the same. The back boxes do appear on eBay for around £100 per side from time to time. Failing that, a custom made cat back system can be bought for £300-£400.
Suspension ball joints and bushes. These can turn costly from Lexus, some aftermarket ones are ok, some are rubbish quality. Some of the bushes are available seperately aftermarket to overhaul the arms, rather than having to buy the whole arm (Lexus only do complete arms). There is a link in my signature to a tutorial I did for changing the front top wishbones.
Sticking caliper and/or slide pins. Very common on many of the older Lexus models, not just the GS. There are a couple of my tutorials in my sig around the the calipers and slide pins.
Sticking solenoid on starter motor. Again, quite common on a lot of older models. Again, a tutorial in my sig for overhauling the starter. Actually getting the starter off is a pig (lack of space), but if you are in the trade, I guess that isn't a concern.
The 10" rear wheels are prone to cracking the inner rim. They can be welded, but just an added expense on top of a refurb price. Two out of the three sets I have needed welding (3 out of the 6 rears)
Rear crank oil seal. A common'ish problem on the 2JZ-GE engine. Gearbox off job to rectify so can be costly, but helped for sure if you are in the trade.
On a long steady run on the motorway, mine used to get 32 to the gallon. Around town I averaged low-mid twenties, if you are a bit enthusiastic, then mid to high teens.
No idea on comparisons. Power is good, expect it a teeny weeny bit down on the book figures as the big sport wheels look the business, but do add to the unsprung wieght a bit. Plenty of get up and go regardless though. Comfort is great (not as great as the later mk2's, but still great). Handling is OK, but I would expect the handling of your other two to have been better (my only BMW experience was my old E36 £25 Coupe - whose handling was fantastic).
As long as it has been maintained well, you shouldn't have any problems. The 2JZ-GE engine is a very strong unit.
Good luck if you decide to go for it. With the 10" rears, they take some beating for looks and stance.