Those are very cool cars. I've owned two on that platform ('93 Cougar n/a 3.8, '93 Lincoln Mark VIII) and I liked them both. I almost bought a '89 Cougar XR7 supercharged/5-spd (RARE) but it had a leaky HG and I couldn't afford that immediately after buying the car.
My biggest beef with that platform is that it's really, really loose. It shudders on rough roads, it sounds like it's gonna break in half when you do something like make a sharp turn onto an uphill driveway, and the doors stick/jam when you jack up the front end. They do handle quite well for their size and have excellent safety ratings in spite of that, though. I especially liked how the Cougar handled-- it actually had a very stiff suspension and with the light V6, it was extremely neutral. I annoyed more than one Porsche/BMW driver on twisty roads by keeping up with them in a bigass Mercury.
The ABS problem is most likely a sticking $5 pump relay or a bad pressure accumulator (~$100, common with age/mileage), although it could also be the $1500 module itself as Phil said. When any of those fail you have no power assist and no pressure at all to the rear brakes.
One quick test is to firmly press the brake pedal repeatedly with the car running. You should be able to do this at LEAST 3 times before you hear the brake pump kick on. Any less, and the accumulator is weak/dead. If the pedal gets hard and/or any brake lights come on before the pump turns on, it's a sticking relay or the pump itself.
Ford beefed up the 3.8 and reworked it just enough that the supercharged version blows head gaskets about as often as the n/a version. I've talked to a very knowledgeable senior tech at Ford as well as the former service manager at a Lincoln Mercury dealership. They both said that the newest multi-layer steel head gasket design helps quite a bit, but the engine has a basic design flaw which means nothing is a 100% guaranteed lifetime fix.
Nonetheless, it's a cool enough car that I think it's worth pursuing. Good luck!
