Hello mate. Sorry to hear about all your problems.
I imported a Toyota Hilux Surf from Japan back in 2005. I still own it, but I've had to replace the radiator, both batteries, starter motor contacts, front discs and pads, front drive shaft CV rubbers, rear shocks, mid exhaust box(once) and rear box twice (2nd time yesterday) and the clincher, a f***ed head. I'm lucky in that I can do all the mechanical work myself, but it's cost me nearly £4,000 in repairs since I've owned it. Add that to the purchase price and I've spent a fortune. No one wants these trucks anymore as they only do 23 MPG. It's sorted now as I've done all the work to it.
What I'm trying to say is that these things happen. You can't predict if something is going to be such a lemon. I want to buy a F800GS. Some posts from owners paint this bike to be very unreliable with constant problems. Other posts from owners say they have done over 50,000 miles and they haven't had any problems.
I would take the Adventure and dump the previous bike in their lap if your getting the same or better deal. You haven't mentioned how the Adventure compairs to your GS. Don't accept a higher mileage bike that's older just because it's an Adventure. Tell them because of all the inconvenience you want a newer, lower mileage bike.
I'm sure the lack of ABS wont be a problem. It's more to go wrong. Bikes never used to have ABS and people still managed.
You can always ask for a full refund and buy a different make, Tiger 800???