As I mostly now live in Bulgaria I have done the trip many times, my normal route would be via Serbia, as they have really good peage style roads, but if you are carrying animals on the European ‘passport’ scheme it will have to be via Romania, again roads have improved dramatically over the last ten years, Buccharest ring road excepted. Then from either Serbia or Romania it’s Hungary (expect border delays!!) Austria, Germany, all the way across on the A3 and A4, again massive road improvements with very few delays. As I do the trip a couple of times every year I also know where to buy fuel, for example last year diesel could be bought in Austria for €1 (£0.88p) per liter, todays price in Bulgaria is £1.01p a liter. If you need any more detailes info give me a shout.