Don't rush into purchasing; allow yourself a cooling off period if you see a must-have property. Prime strategic location is one of the most important factors.
Do not sign a contract in a foreign language unless you are completely satisfied.
If possible try to arrange your mortgage finance in principle, before agreeing to purchase the property, or before signing any contracts and paying over a deposit.
Consider combining your financial resources with friends or family, it could enable a villa with a pool to be within your financial reach, rather than simply an apartment.
Check with the Estate Agent that you are aware of the costs charged by the government and legal authorities for purchasing a property in Bulgaria.
When buying residential property consider seeking specialist advice (i.e. solicitors, etc) before committing to a purchase overseas. They should be proficient in the country's laws and processes and know the specifics involved in buying a property there. They should also ensure you don't inherit any debt on a property before you purchase - i.e. if the developer has borrowed money to build the development and this amount has been allocated against each plot as additional security to the developer's bank.
Set up standing orders in a local Bulgarian bank account to meet bills and taxes.
All new build property in Bulgaria is subject to 20% VAT.
If you will wish to rent your property, then ensure the company you purchase through is able to provide that service and is able to manage and promote it properly on your behalf.