It seems that the real problem is not the phone (you could buy it without mobile plan) but the data plan. I am sure that if you were already an Mtel customer they would allow you to buy what you wanted.
The best (cheap and convenient) solution for you is to find a friend (company, your employer) with MClass plan and with some spare SIM card. The only problem stays with the owner of the plan - he could not decrease the number of the SIM cards in his MClass group. So if you leave Bulgaria he has to continue the monthly payments (for minor plan of course) for the extra SIM card.
BTW Mtel gives good monthly offers (mobile and smartphones) for MClass customers.