The Bulgarian summer is not - in my opinion - short. It is pretty much wall to wall sunshine for 4 months. The problem is that the only time you will get rentals (if you get any at all) is when the kids are off school. Which narrows it down to about 6 weeks max.
I have been renting for 6 years. My advice would be - as everyone else says - forget it.
Even if you buy a property as an investment speculation, what most people seem to forget is that the property will need maintenace and upkeep every year that you don't sell it - and believe me, unless you don't mind your property falling apart, yearly maintenance can be pretty high.