I've been reading here for awhile, but my experience is limited, so what I'm about to say is a distillation of advice given on here by others, a tl;dr, as it were; and a bit of opinion from me, especially the last two paragraphs.
Since you asked about an inkjet there are two ways to go - inkjet transfers, using pigment inks; and dye sublimation. In both instances, start with an Epson WorkForce WF-7210 (you can buy 7 for the price of 1 R1800), with 3rd party refillable cartridges, and appropriate 3rd party inks. Either way you are limited, for best results, to white fabric; cotton or blend for inkjet transfers, polyester for dye sublimation. JPSS paper for inkjet transfers, I don't know about dye sublimation paper, need to do more research.
Or, you could buy a good heat press and have someone plastisol print your designs (screen printing on heat transfer paper).
You can buy a cheap, no-name, Chinese heat press from ebay for $200 or less, and you're on your own, or a Chinese made press with some dealer support for around $450. If you want the top end, a George Knight DK20S for something less than than $1500, or a HIX SwingMan 20.
Dye sublimation gives bright, vibrant color, which might be better for reproducing art, but you'd want to be careful about color matching. But for another $200 you could also have a dedicated pigment ink printer (best not to switch inks in a printer) and use whichever suits the specific art. Throw in a Silhouette Cameo for $200, to contour cut your inkjet transfers (again, pigment ink on JPSS paper), and the occasional heat transfer vinyl job, and for less than the price of the R1800 you can be up and running with both inkjet transfers and dye sublimation, and upgrade or replace equipment when the need arises/your t-shirt business grows.
That's the down-and-dirty, testing-the-waters, startup business plan. Not much invested, not much to lose. If you've got saleable designs, and you already have business experience, you've got 90% of it, the rest is just "manufacturing".