Nick,
The cheapest investment for you would be to start out with digital transfers (inkjet for cotton and sublimation for polyester) or using a vinyl cutter (which will require you to weed the design from the heat applied film). Both of these techniques have a heavier print on a shirt compared to
dtg printing. So you really need to determine if your customers (i.e. target market) will purchase a shirt done as a digital transfer or heat applied film. If so, that might be the best way to start your business of until you can grow it to a steady revenue flow.
Depending on what you are getting paid for the shirts sold for one at a time, it might be worth it to invest in a
dtg printer. You could look at a used printer as well.
dtg printers excel at short runs and the number of colors is obsolete. I would not recommend creating a screen to print one shirt. Besides the extra time and labor, you will have to clean the screen (the dirty side of the business). Just does not seem worth it to me for 1 shirt. 12 shirts, 2 color or even 6 shirts, 1 color... then I would consider screen printing.
Just my opinion,
Mark