i've found that with just a heat press, you can make great grilled cheese sandwiches, bacon and hamburger patties. j/k.
just doing heat transfers may be a tough gig. generally, people want more even out of a cheap shirt despite being able to get almost an unlimited range of colours onto a transfer. the problem is just the layer of film it leaves behind. plus, i'm not sure how you do white on a dark shirt (i'm assuming it's some kind of underbase, but i'm not sure as we've never done white. it's usually not a problem as it's meant to be a rather cheap process and, that being the case, talking the customer into a white shirt has never been an issue). we've done small runs using transfers for certain things, and small runs is really what transfers are good for. the quality is usually pretty good. they don't crack, peel and fade like the old kind like when i was a kid. typically, the border that's left after you trim around the design doesn't show up *too* bad.
something to consider is doing plaques. on silver, white and brass (usually we use gold aluminum instead as it's brighter), you can do some really cool images. if it's for nametags, we use cover seal from magictouch to help protect it. the process for putting a transfer on metal is just like putting one on a shirt, though you'd need to adjust your temp and time possibly. you can make your own plaques or be a trophy shop's outsourced supplier. depending on what the job is, our profit margin on a plaque is upward of 300%. last week we just did 75 dash plaques for a car show using transfers, so the work is out there. we only charged $2.50 @, but it's not back-breaking labour and we may have had $20 in materials, if that.
the nice thing about transfers is the ease of production. the bad thing is the quality is on the meh to okay side and is rather a 'cheap' option. i mean, someone that wants a hundred colours probably doesn't want to pay the price of a four colour process on four shirts, lol. $40 for that shirt plus another $25 for set-up (i'm being cheap here) or $12 with a transfer? it's up to the customer and their wallets to decide.
you might do okay at a flea market with transfers. joke sayings for $6 a shirt, or some lady's pet pooch may pay the booth rent, who knows?
with just your heat press, you can have plastisol transfers made, which essentially are just like screen printing. the transfers are screen printed, sent to you, and you heat press them onto your substrate. there are folk that use this as a business model and pretty much own nothing but a heat press and order shirts as they need them.
it's difficult to say what your best options are, of course, but you might want to check into vinyl, too. there are tons of things you can do with vinyl.
practice your photoshop. other programmes to check into are corel and illustrator.