You will have to do your homework on this one.
If you decide to go the purchase route. Any
DTG that prints on dark color has to be run very often (daily). Some machines are required to be running 24x7 to keep the white ink mixed so the solids do not settle.
The dark color garments have to be treated before they can be printed on. If done incorrectly, the image will wash out during the first wash. Ask for a sample of what the machine is capable of doing print wise. Check it out carefully. I found some to be far worse than screen printing ever could be.
Print heads are an expense item you are told about upfront. The titanium in the white ink is very abrasive and wears them out fast. Cost are somewhere around $400 a piece. The only machines that use print head design for the job are Brother and Kornit. You get what you pay for.
I say contract your work out or do heat transfers.
Good luck.