If it was me, I'd actually book an experienced gay escort! Pay him for his time (a good couple of hours), and tell him exactly why you're booking him. He'll know exactly the kind of thing you'll be expected to do as an escort, and you can try those things with him. That way, if you hate it/can't do it, you're with someone who understands and it's not an awkward situation.
I think it's a worthwhile investment. Your other options are to just pull a gay man on a night out (a bit cruel in my opinion), or to pitch up for a client and hope for the best (like Pandora said, could turn nasty if you're not able to manage the booking, esp if the client's forked out for a hotel).
I think you need to know if you can do it before you take a booking on. Us ladies are lucky, a bit of lube and physically we're good to go, whether we're feeling it or not. You've got to get it up and keep it up! I'd rather lose a couple of hundred quid finding out I can't do it, than risk getting my head kicked in by an irate punter (or worse).
Keep us posted! xx