Flights are not included in the rental cost but can be bought from many airlines including, bulgariaair,  wizz air  and easyjet or try the normal lastminute.com etc. - prices start from £40.00 each way inc taxes per person with easyjet (Gatwick - Sofia)

Transfers from Sofia airport to the house by mini bus can be supplied at a cost of around £80.00 per party of up to 10 people and take around an hour. ( cheaper transfers can be arranged for under 4 people)

I would recommend hiring a car in the summer to see the sights but making use of the transfers in the winter.