I am not a Bulgaria expert but my experience is that there are very few on this board. Try doing a search. I remember we have had a few threads before.
Don't sweat the hotel in Sofia. Sofia is a wonderful city and the Bulgarians are generally friendly and helpful. Limited English is spoken.
Food is amazing: a combination of Greek, Turkish and overall Mediterranean food and flavors. If you like eggplant, peppers, fresh ingredients and lamb you are in luck.
Must have rakia (a grappa like drink) with shopska salata (tomatoes and bulgarian feta cheese (much nicer and creamier than the Greek version).
Wine is actually not bad, Melnik is the better area for wine but wine comes from many regions.
Bulgaria is famous for its yogurt.
I think 2-3 days in Sofia should be enough. One or two days outside to Rila Monastery, there is skiing close to Sofia if you go in winter and Veliko Turnovo (the old capital with medieval city layout).
Moving around Bulgaria may be difficult on your own unless you really like to venture out. (my family speaks/understands bulgarian so we rented a car and ventured out). Plan on taking a tour. I think it is worth it.
If you want the beach, the whole area between Burgas and Golden Sands is packed with resorts and towns around the black sea. Be careful. Many all-inclusive places with tons of people, a lot of young 18-25 crowd looking for a lot of food and cheap drinks make the area less fun.
Varna is a very gracious city, great pedestrian area, tons of restaurants around the waterfront. We stayed at the Musala Palace Hotel. Unbelievable service. Google it. I think the coffee menu had 20 different coffees and the breakfast was an elegant feast. Worth a try. Same hotel in Paris would easily cost 4-5 times the price.
I would buy some rakia, great lace products (look for hand made), rose water (Bulgaria is famous for the roses) and some great flea markets that sell second world war and Soviet memorabilia.