It's a tough call considering the mileage. There's quite a few "wear items" that could rack up the bill. Assuming everything else is in good shape, and you can do the timing chains yourself, I would probably go up to 5k. You have to really want the car, prepare for an engine over-haul, and pray the cylinder walls aren't damaged. Honestly with piston rings being such a common failure at that mileage, and you can't drive it because of the chains, probably best to shoot for 2-3k. The seller can't ask for much since the car can't be driven, and timing chain service (if paying a shop) exceeds the value of the car.
At 150k you could expect certain things. Other than normal wear items, these cars have particular problems to address. Upper control arm bushings, front and side engine mounts, driveshaft center support is probably sagging, Slave cylinder boot could be melted (saw it on my b6 with 195k and the other with 80k, damn precats), dual mass flywheel loose (again, both cars), piston rings, valve stem seals, oil cooler seals, a/c compressor, aux radiators, pcv, intake runner, etc.
Some of these things can be expected, if there aren't service records provided, you could subtract the costs of expected repairs from that 5k. Then do a thorough inspection and subtract those things.
Obviously many things can be done over time, but you should factor in expected repairs. I'd give my left lung for a clean avant, so the little things wouldn't bother me, but you should be concerned for the engine mileage. Good luck!
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