TCs, SCs, and XR7s are cool. My best suggestion is to go right to the guys who breathe for these cars on the turbo coupe sites. Do your homework, as there are not a lot of pitfalls, but you need to know what you're doing to get the most from them. They are heavy, but will out-run a stock 5.0L version, so they can move. That raises another point, in that a little fly-weight import with a turbo engine nearly as big will clean-up on you, so decide now if awesome power or fun economy is your dream. You know, you can turbo or SC the 5.0L version too
but you also know with a heavy foot your final MPG will be a bit ugly.
Upgrades are relatively cheap and easy and there's lots you can do, but be aware the faster you go, the more expensive it becomes for a DD. Also the mileage is not significantly higher than the V6 and V8 versions. Like I said - it's heavy.
While a lighter car would do better with the little 4-banger; if you're stuck on the style I would consider the XR7 as they aren't abused as much and carry a significantly lower price tag as many are not even aware of the turbo version. As they are nearly identical as a Mercury sister product, all the tips that apply to SCs apply to them as well. Change the badges to SC if you like.
I would avoid buying a car with an upgraded turbo. It's not a big deal upgrade, and it can be done well or be crap. Most of all, it tells you the PO was obviously pushing it and the rest of the car hard. Most of the time that's not a good sign for buying a used car. I would go find Granny's beautiful XR7, give it a wax job and start having fun - but that's me. Buyer beware.