I used www.bulgariacarrent.com
last week. Excellent price and excellent service. You don't choose the model, you choose the type
of car that you want, eg mid-sized, full-sized, 4WD etc. I only needed a runaround so I chose the 'unspecified' option. I ended up with a very nice late model Corsa saloon, delivered to Varna airport. 105 Euros for one week. That's half the price Horses paid!
You can book it all online which is a boon. In addition, for a small extra charge they will deliver and collect from Bucharest, which means that next time I'm going to take adavantage of Easyjet's cheap flights from Luton, so Bulgariacarrent.com really will save me a wedge!