Hi we have been to Borovets for the last two years and have used the two atm's that are in the resort.
we usually get a better rate on the atm!
we always take some english cash just in case but its quite cheap out there so you dont need alot of money as a guide we usually spend about £300(between me my wife and my 6 yr old) and thats a struggle
hope this helps
There machines around the resorts but you will be charged a fee each time you use them.
A sensible amount of money depends whar sort of party animal you are in the evenings!