Hi Bob?
The ba threads are by far the most common here in model engineering but you will also come across 26, 32 and 40tpi ME threads although I think the 26tpi are actually "cycle" threads.
I bought a ba set, quite possibly from RDG I can't remember, and haven't regretted it.
Don't forget you will also need all the relevant drills for tapping and clearance holes - again a set is probably best value, especially if you intend to continue with this.
Be warned, you willl amass tools far faster than you do models.
My best and most used buy wasn't a tool, 'twas The Model Engineers Handbook by Tubal Cain - essential as far as I am concerned.
Cheers, Andy