Seeking Arrangments (SA) is largely the way to go. Not perfect, but the best site out there. And there is 112 pages of thread discussion that largely affirm that. The fact that you were already talking about "buying credits" tells me you are starting off on the wrong foot. Engage these girls without giving them online gifts. If you start doing that, they will peg you as a sucker immediately. If they demand it anyway, drop them.
But, hey, TOFTT and let us know how it works out.