congrats on blowing your HG, it's the fun part!
Also check your oil, does it look like a chocolate milkshake? Some cars have that happen where the antifreeze gets into the oil. Mine didnt, but it never hurts to ask!
Well i'll try to answer your questions here for ya.
1) you will totally need to pull the heads completely off the block.
2) just follow the haynes manual for head gasket replacement, thats exactly how I learned how to do it.
3) Gasket kit was about $110.00 for me when I purchased it, and it comes with everything you need, new upper and lower intake gaskets, head gaskets, throttle body gasket, EGR tube gasket, etc etc.... Even came with valve-stem seals.
4) As far as I know the thunderbird/cougar kit is exactly the same as long as it's for the 3.8
The mustang engine is the same also, so I would assume that it's gonna be the same.
5) Rough estimate? Well when I had mine done, it was only about $100.00 and that included having them checked for cracks, milled, and a valve job done. Highly recommend the valve job, it will definately help out, and you got the heads off so might as well do it!
6) I cant tell you honestly. 3.8's are notorious for blowin HG's. I blew mine on my 96' at 102,700. And now I'm almost at 111K on it, and no problems absolutely at all. Just make sure it's done right the 1st time and you should have no problems what-so-ever.
Remember, if you have any questions on here, just feel free to ask! There's a plethora of knowledge out there, and HG's are one of the most common things on here to learn about.
Enjoy having fun with it. It took me 3 months, but that was because I was moving, going to school full time, and workin 40 hours a week. If I knew then what I knew now, i could simply do the HG's in a weekend. Once you do it the 1st time, it makes it just that much easier if it happens a 2nd time.
Also one quick note. IF you do it yourself MAKE SURE you get a Torque-to-Yield indicator. That will help you so much with stretching the head bolts on the engine.