@cowanmaureen
If your budget stretches, perhaps a flat in a large city (Sofia, Plovdiv, Varna, Burgas) would be quite convenient? Bansko (Bulgaria's largest ski/mountain resort) is very charming too, and there are some lower-priced flats still available here.
If you want a house, then I'd recommend villages within about 10km or so of large towns, if you can. It depends on your budget, of course, as the larger the town/city, and the closer your village, the more expensive it will be.
Our village house is 10km from Kazanlak, which has a population of 70,000. Kazanlak is a nice town with a Rose Festival and all standard amenities. The village has good bus service, maybe 5 times per day, and it takes about 45 minutes (2 leva). I'd have to spend a couple of hours in town to get the bus back, or I can grab a taxi for 10 leva. To compare, we drive, and it's 10-15 minutes in a car, and you can go/return when you want. These car things are quite convenient, I believe they might catch on. :-) But I've caught the bus a few times, and it would be painless enough if I were a non-driver.
Some people live further away, but then a trip to the supermarket does kinda turn into a full-scale mission. :-) Depends what you feel comfortable with.
My guess is that anything over 50,000 will have hospitals and cafes and good supermarkets (Lidl, Kaufland, Billa). It's good to have civilization close by when you need it. :-) This would give you plenty of options. If you think your town needs to be 200k, then that narrows it down a lot. If you think a small town of only 10k-20k is good enough (should have basic amenities, including a small market) then there are lots of options including Pavlikeni, Polski Trambesh, Chirpan, Elhovo and so on.
There's a list of cities in Bulgaria by population at the following link, so you can see the full list. The biggest are the major cities listed above. But I like our town (Kazanlak), and nearby Stara Zagora is even bigger and nicer. Veliko Tarnovo is a lovely city, and villages nearby are very popular with expats. Gabrovo is next to the Balkan Mountains too and I think it's very nice. I like Plovdiv district a lot (it has Balkan Mountains and Rhodopes) and its 3 cities are great (Plovdiv, Asenovgrad, Karlovo). Shumen, Ruse, Yambol maybe?
Down south-east is cheaper, but more sparsely populated, so you might be further from a good supermarket, and the bus service might be a bit more infrequent / non-existent. In this area, the towns I'd suggest are Haskovo, Svilengrad, Elhovo, maybe Topolovgrad.
Blagoevgrad (or Razlog) is near Bansko, but bigger, so this might be an option too.
https://en.wikipedia.org/wiki/List_of_cities_and_towns_in_Bulgaria