It s an open ancient theatre.

есть еще на ул отец Паисий магазин музыкальный там тоже продают но чаще всего билеты раскупают заранее

I used Dinars in Serbia. I have not been to Romania; only saw it across the Danube river.

Sorry, I don't know, I used my debit card in ATMs to access local currencies all across eastern Europe with no problem. If you do try to use dollars or euros, they may accept them, but you will lose on the exchange rate they use.

I haven't seen a ramp for wheelchairs and in general the access to the place to start with will be very tricky - the old town is mostly laid with cobble stones and on foot it can be tricky, let alone with a helping vehicle. But you can always call the tourist info centre at +359 32 620453 and they should know best.