Hi Helen!
We bought through Bulgaria Direct, so I can comment on how their pay-monthly works. A number of other sellers and property sites seem to offer the option, some may be more legal and reliable than others.
The way Bulgaria Direct do it works like a rent-to-buy scheme, not a loan or a mortgage. There's what appears to be a legally robust (under Bulgarian law) contract stating all costs and the rights and responsibilities of both buyer and seller. You get full rights of use of the property as soon as they get your signed contract, and you also assume all responsibility for the property, condition, bills, etc from that date. The property doesn't become yours until the legal transfer of ownership is arranged, after the final payment has been made. Payments are easily made online via direct bank transfer.
They don't require proof of income, or do any credit checks. But as Jim said, if payments are missed, the rights to the property revert to them and everything the buyer already paid is forfeited. It's likely they'd be understanding in special circumstances if they were notified in advance about difficulties making a payment, but buyers do need to be sure they can meet those monthly payments before committing to buy. I'm sure you'll verify that first.
The house we bought was in slightly worse condition than I anticipated from the description and the photos. A massive leak in the bathroom roof gave a bonus shower every time it rained! Because of the roof design plus damage to the roof timbers elsewhere in the house from past leaks, we decided to get the entire roof redone rather than try to get the back section patched up. That was a big expense we hadn't anticipated having to spend so soon, but the guys did excellent work and also were able to raise the ceiling height in that section for us as it was designed for people under 5' tall! Realistically, most village houses, especially ones for the low price we paid, will need work done sooner than anticipated. Ideally, look at the house first before signing the contract or get someone else to go look at it for you, but if that's not an option, as a minimum ensure the photos are recent. We're happy overall with the property and are impatient to move there, would go tomorrow if we could.
Pay-monthly worked well for us. We could have paid cash, but doing pay-monthly allowed us to spend that money on the new roof, instead. Danislava, Bulgaria Direct's admin person in the VT office, is wonderfully helpful. The process of transferring ownership after paying off the house was smooth and easy.
I'm not sure how long it is since you were last in Bulgaria. Prices for everything, from property itself to groceries to building materials to the costs of employing tradespeople, have risen a lot over the past five years. If the house needs renovation work, it may cost more than you expect. Based on what we'd been told to expect when I first seriously wanted to move to Bulgaria, about ten years ago, I'd anticipated needing to spend as much again as we paid for the house. In reality, it will be nearer three times by the time the house is done. But that depends on the house and the purchase price. As they say, buy cheap, pay twice. It's still way cheaper than Britain.
We have zero regrets, besides wishing we'd done it sooner! Unfortunately I wasn't able to get to Bulgaria before the Brexit cut-off date to do the residency application, and hadn't realised just how difficult it would get post Brexit and Schengen. So now we need to wait till I reach retirement age. Missing that opportunity when it was still possible is my one regret! Buying the house we did, the way we did, I don't regret in the slightest.