I had one for a couple of years. First things first:
www.sccoa.com. Great source of info.
A few points: One, it ain't an SVT. You will be driving two tons of fun there. Two: they chug premium gas. Mustang 5.0 mileage, and it drinks premium = $$$$. And as it is supercharged, don't even THINK about running regular gas.
And about the head gaskets. They are notorious for going on regular 3.8s. These are pressurized, and it makes for a bad mix. Ask him if he knows what brand were used. Fel-pro makes a gasket that pretty much eliminates the problem. Ford used it after 94 on ALL 3.8s.
As for the ABS system, given his aking price, I would guess that it is the control unit. It's a Teves unit that costs about $1500, IIRC. Or you can go to prioreman,com, and get it WAY cheaper.
Check the shocks. They are electronically controlled Tokicos, and cost a pretty penny if you want to keep the electronic cotrol.
If you have to face winters, you will NEED snows. 315 foot/lbs of torque will do that.
If you buy, change out the resonator. It was designed to be restrictive to keep the car from waxing the Mistang on the track. Some guys claim 15 at the wheels with this simple mod.
They aren't too hard to work on if you know your stuff. Again, SCCOA is a good source. And they "fly" under the radar of the cops for the most part.
I had an 89 and loved it until my clutch hydraulics went. The shocks went soon after. It more than it was worth to fix it by that point, and now I have my SVT. But I still check the Auto Trader for them every week.