Is it possible to take a regular washer / dryer and put a coin-operation part on to it? If so, what's the cost in doing so?
I don't think so. Your best bet is to either purchase a coin-operated W/D (I'd start on Craiglist) or you can "rent" one. If you do the latter approach, there shouldn't be any upfront costs and you won't have to pay for ongoing maintenance. But whomever you "rent" from will take 50% of the profits.