My boyfriend worked for a screen printer for years and has done shirts for some of our friends bands in the garage. From what I know, the main thing is to get professional quality supplies. Ink, screens, blocking agent, etc. The shirts wind up looking pretty half a**ed if you use the stuff from the craft store. Other than that, start up can be very inexpensive. You can make 1 to 2 color shirts for businesses and stuff without having to invest a huge amount of money on the big expensive equipment. You just need the shirts, supplies, and the ability to create graphics on the computer....
I would invest in doing screens rather than investing in the heat press and transfers. Doesn't make sense to me to start down a path unless that's the direction you want to travel in. Just my 2 cents...
If you have any other specific questions let me know and I can ask the bf when he gets home from work.