Hi Judy
Anywhere in Bulgaria can get cold in the Winter, even the Black Sea has been known to freeze!
We're quite lucky as we're in a small town with all facilities (hospital, shops, bars, restaurants, markets, etc.), but surrounded by mountains.
You may wish to investigate Sandanski region, the warmest in Bulgaria! That's right in the South West, close to the border with Greece but in the Rhodopi mountains.
Sliven, in the central area is surrounded by the Balkan mountains (but gets colder than Sandanski in winter).
Here's an article about Haskovo town:
[url=http:
//www.
questbg.
com/en/area-focus/cities-a-towns/705-haskovo-city-of-the-blessed-virgin-mary:1n1taqjg]Haskovo Town[/url:1n1taqjg]
And here's another about towns and villages around Haskovo:
[url=http:
//www.
questbg.
com/en/travel/out-a-about/730-beyond-haskovo:1n1taqjg]towns and villages around Haskovo[/url:1n1taqjg]
I think at the end of the day you should figure out the main priorities that are important to you and look at places that fit the criteria (for instance do you want to be near facilities, near an airport, near a hospital, etc.).
Good luck with your search, one things for sure, Bulgaria has plenty of mountains to offer!