Good timing!....I have just arrived back in Bourgas 2 days ago from the UK driving a Land Rover Discover,having travelled over to Scotland just before Christmas!!
I travelled back from the UK across the Channel, Dover to Dunquerque, 44 Euros, with DFDS Seaways, then my route back was Belgium, Germany, Austria,Hungary, Serbia, very simple route and motorway all the way to NIS in Serbia which is around 100km from the Bulgarian Border.
You will need a green card if you are going the Serbia route, I have done the Romanian route but find it longer, the roads in Serbia are excellent.
As the other poster said plenty of screenwash, the UK stuff doesn`t really have the strength so use undiluted, I also treat the glass with RAIN X from Halfords which is really helpful, and carry several bottles of water in the car.
You will need high vis vests,(must be within reach when in Austria) warning triangle, spare bulbs, shovel, snow chains, fire extinguisher and first aid kit, there are different rules in each country but that will cover everything.
I also carry spare belts for the engine, spare wheel bearing kit and a spare waterpump, these items are the most likely to go on a Land Rover Discovery and they are really cheap to have in the back just in case.
I also have euro breakdown cover too just in case.
I would also invest in a cheap satnav, it`s a simple route but less stressful with satnav help.
There are Vignettes to buy for Austria, it`s a sticker, for Hungary, you pay and get a receipt, keep the receipt!, the cost is Austria 13 Euros for 10 Days , minimum and Hungary 4 Euros, you can buy both from virtually any petrol station near the borders.
There are loads of rest areas for the entire journey and lots of motels by the motorways many around 10 - 15 euros a night.
The motorway in Serbia has three toll sections which works out around 15 euros in total, there is no other vignette to pay, if you go the Romania route you will need a vignette there anyway.
If you want any more detailed info pm me and I`ll be happy to give you more info.