Stock Stall speed of a 98+ Mustang TC:
Min RPM: 2324
Max RPM: 2737
Depends on the SC type you go with. A 3000 RPM is ok but likely a little low for a Roots or Twin Screw setup. For a Centrifugal SC, you would want to have a 4200 RPM TC. Getting a TC with a higher stall than 4200 RPM is not going to be easy, so expect to spend Big Money if that is not high enough.
As for a good after market TC, these guys are real good and have a good Single Disc TC:
http://www.circledspecialties.com/
http://bc-automotive.com/
If you look at Precision Industries( PI ) TC, only consider the Triple Disc. The Triple Disc is an ok TC but uses crappy lining, so that will wear out over time.
I currently run a PI Triple Disc that was setup by Chris at CircleD Specialties. IMO, that is the best TC available to you right now which is a properly setup Triple Disc.
For a mildly modified car, a good Single Disc TC will be ok. For a more heavily modified car or power added car, the Triple Disc is the one to have.
I would recommend nothing less than a 3500 RPM Stall TC. The other TC's available that I know of have some downfalls that I don't like in a TC, that is including Edge or West Coast Converters that you have seen advertised in this thread.