I hate to be the one to say it, but that's exactly what "as-is" means . . .
If you bought from a dealer, they should make it right. If you bought from a private party, well then I'm sorry.
Also, I've asked that this thread be moved into the
Other K-Bikes Discussion
forum, where you may get more model-specific help with your Flying Brick GT.
As you've already discovered, common issues with older ABS bikes are weak battery causing the warning lights to come on, low fluid causing the warning lights to come on, worn pads causing low fluid and warning lights. You could also have old, gummy brake fluid causing the internal ABS solenoids to stick. Or it could simply be a broken wheel speed sensor or wire, which would also account for the lack of speedo.
So your proposed plan is correct: charge battery, inspect wheel speed sensors, and completely flush out all old brake fluid. There is a set of instructions for how to do this on Brick RS/GT bikes, but I don't have that link handy at the moment. There may also be a guy who can rebuild these older ABS units, but again, I don't have a direct link.
After all of that, see where you stand and maybe we can offer additional advice.
As for the ABS debate, I've ridden the last 300K miles on ABS bikes, and I won't buy another bike without it, period. If you never need it then no big deal, but the one time it saves your ass it's worth whatever extra you paid for it . . .
Good luck, and thanks for your service.