Hi,
Bankso is getting a lot of good press lately and a lot of investment has been pumbed into it. The skiing is meant to be good and facilities have improved. However, I would say it is high risk. I was in Bansko with property developers about a year ago and I was amazed by the amount of construction going on, the place was a complete mess. Last week a friend of mine was in Bansko last week and was amazed by the amount of construction going on!!! There is no limit to the amount of apartment complexes they can build in bansko because there's thousands of flat land around the resort. They are certainly building much quicker than the number of tourists so there's an oversupply of accomodation. I'm not sure how things will fare out in Bankso but I'd worried about investing there. If you're really keen on a ski property in Bulgaria I would recommend that you travel over to Bulgaria and visit Borovets, Pamporovo and Bansko (i.e. the 3 main resorts). You could visit all 3 in a week.
If you are considering coastal properties I think the region between Varna and up along balchik (the northern coast) is a great area. There are 3 international golf courses currently being built just past balchik - 1 Ian Woosnam course and 2 Gary Player courses. Golfing season can be up to 9 months of the year.
If the developer is offering guaranteed rental just assume you are paying for it in the price because you are!!!