I'm in the process of (hopefully) buying - our first visit is next week. You have to have a company to buy a property that is on a piece of land or to buy land, ie. a house.
I'm looking at off-plan apartments and you don't have to have a company to do that.
You need to set up a company in Bulgaria if you are purhasing land , if you purchase an apartment you don't have to because the purchase doen't invovle land. There are no exceptions if you purchase a property with land you need to set up a Bulgarian company. However this is not difficult , takes up to a month to complete and the process goes smoothly .
Good luck and happy investing !