@Raizori
I can only really speak for Varna and further North. I live in Varna, which, itself, has beautiful beaches, parks, shops and businesses for absolutely everything you could want and it's twenty minutes from rolling hills for hiking. However, it depends what you want. If you don't like city life then Balchik is certainly a good compromise, as it has enough infrastructure to live year round. Balchik is a seasonal place, very busy from May to September, then dead in the winter. That said, all the coastal towns are pretty dead in winter. Balchik offers the rolling countryside and hills too, has a small range of shops and businesses and isn't too far from some secluded sandy beaches. It's forty minutes drive to Varna and there's a bus service to/from Varna too. A taxi to Varna is around 75 Leva.
You may want to also consider how close you want to be to an airport. Both Varna and Burgas have airports. Burgas boasts more charter flights in the summer. Varna Airport has a decent selection of year round flights, including Sofia, Vienna, Istanbul, London, Brussels, Paris (new) and many German cities.
If you want something even quieter, have you looked at Kavarna, another half an hour up the road from Balchik. Kavarna also has a beautiful sandy beach, although the town itself ,is a kilometre away from the coast. However, Kavarna is flatter, more for farming.
I should also mention the climate. Winters are bearable on the coast, with very little snow, compared to inland areas. The temperature rarely drops below zero. However, we did have some heavy snow in Varna this winter, but it disappeared in two days.
Also, you may want to consider medical help. Balchik has a basic, regional hospital, with Varna having many hospitals, clinics and medical centres. Kavarna also has a basic hospital.
You will also find the roads quieter between Varna, Balchik and Kavarna, with very little traffic, even in summer, which, in comparison to the Varna Burgas road, is a godsend. The Varna Burgas road isn't for the faint hearted in the summer months!
That's my perspective on Varna and northward. Sveti Vlas southwards I'll leave to someone else.
Personally I'm biased towards Varna, compared to Burgas, but that's up to you. Burgas is a neater, tidier city,