Welcome!
Maybe we can help each other, I live in MI too... Will PM you my location.
Anyway, I have similar. I just got my 99 though. It ran and drove fine but I knew there were things that needed attention. It has 130,000 miles in it. Here is what I have done so far to mine.
Replaced the broken front sway bar.
Changed coolant, T-stat, up/lower rad hoses and heater hoses.
Changed plugs and wires.
Cut off and changed the fuel filter.
Replaced both headlights.
Replaced the UIM and gasket.
Replaced both valve cover gaskets.
Cleaned the throttle body. It was the blackest I have ever seen too!
Got a few used parts from parts galore.
Fixed a door lock issue.
Replaced the fuel pressure regulator.
Replaced the PCV.
Cleaned the MAF.
Repalced the ignition module. Got it for $14, was not priced correctly and supposed to be $140. Thanks Auto Zone!
Did the battery wire ground mod seen at ZZ performance. Amazing what a good ground can do for electrical! CHeck the site out if you have not, http://www.zzperformance.com/grand_prix/articles.php.
Replaced a few burned bulbs with a few interior switches.
Put on a EVAP selonoid, air temp sensor and washer bottle motor I had, all new from when I worked at GM. Parts I had laying around and probably would have never used if it were not for this car, in the old parts dept!
Buffed the heck out of the black paint, than clay barred it.
Steam cleaned the interior.
Changed the oil.
Fixed the trunk light.
Still chasing down an oil pan leak, got it slowed but need to do a little more.
I am doing all of this, some the average person would not do but I do not want break downs in the middle of winter, or out of state. I put $500 into it so far and and still have to rebuild the alternator, probably another $50 and then it is done!
Since you can do all the work yourself, I would say keep the car. If you could not, then I would say ditch it too. Yours similar to mine, previous owners just did not fix things even if a little item. Now there are many little items to fix!
The overheating and steam concerns me, and I hope the heads did not warp on you but sounds like you know what you are doing though.
