Hi,
If you can, get hold of a copy of the Money (or maybe Home) section in last Sunday's (16/7) Sunday Times - there was a whole section all about Bulgarian property.
Much of the article emphasised that lots of Brits have bought cheap apartments with offers of "guaranteed" rental income to cover the mortgage. Due to massive over-supply, the rental is anything but guaranteed and people are struggling to rent them out at all. Also, many areas are like a building site due to a huge construction push.
My answer assumes you are doing this as a holiday home/buy to let rather than a permanent residence, of course.