Me and my girlfriend toured 10 countries in europe last year. We are no experts in travel or touring but here is our advice and what we done.
1)any newish or well looked after vehicle would do the job. European make would be better for parts but it doesn't really matter much, as long as you have decent European breakdown cover you should be alright.
I went last year in my 97 bmw 318is (E36) which was perfect,
Very reliable car which is sporty, very comfortable to drive and sleep in. also it has lots of space for camping equipment. was great on the swiss alps and felt perfect driving across Germany on the autobahns. It has an mpg reader which helped keep fuel costs down. Theses cars are pretty cheap on ebay. LPG would be an added bonus.
We are going again in a couple of weeks and are debating weather to take the car again or take my 2k ford transit so we can camp in the back, which I think would be better for the longer journey we plan. These vans are also cheap and very reliable and easy to convert into a camper.
Because there was 2 of us toll booths and driving a right had drive car was not an issue. Just remember Bum to Kerb.
2) Berlin is an awesome city, best city for me(take the free walking tour around berlin, A*), rome is a must. we done like a top gear road trip going over all really nice routes over the swiss/italian mountains. Norway has the most amazing scenery.
depending on your combined interests you may consider a third reich tour which is european wide.
I like your route, in fact I might use it for my next trip with a little detour to drive up the Croatian coast, and maybe to Istanbul aswell. And seeing as the route goes so closley to andorra, may as well check that out too.
3) We took All the camping and cooking equipment We never used the tent the car was so comfortable to sleep in.
There are so many good rest stops and large service stations with sleeping areas all over the place(especially Germany and frence)
we slept at them every night and it was perfectly safe. this saved on any camping/ hotel costs. we also stopped at these places to pull out the cooker
and make lunch/dinner. They all had toilets, showers and a tap for water.
4) Obviously when driving in Europe you are required to have things like HI-vis vest/jacket, warning triangle, spare light bulbs and a fire extinguisher. maybe more check that out.
European breakdown cover
Satnav - i used a tomtom go one xl which is 3 years old and it worked perfectly. i also used it for walking around city navigation. i also have garmin on my mobile phone for backup
Map - Just incase
First Aid Kit
Other than these things you should try keep it to a minimum, its so easy to overpack.
I took a big removal crate that has fold over lids which fitted into my boot perfectly with loads more space for food, clothes and other stuff. It also doubled up as a table to cook on
In the crate was:
a small 2 man easy to pack tent. (never used)
a blow up double matteres with 12v pump which plugged in to the car. (never used)
Tefeal Camping satainless steel cooking pots and pans,
plastic plates and bowls.
Small single burner camping cooker with spare gas
18v Makita work light with spare batterys
waterproof over jacket and trousers
We also took a 25 litre water container. Which we filled up at every rest stop if needed.
Multi 12v socket- plug 4 things into the same socket like bed pump, phone charger, satnav, battery booster charger.
Battery booster, which had tyre pump built in and work light, which was useful.
I also had all documents at the ready in the glovebox like insurance, green card, bookings for eurotunnel, etc. which came in handy at customs coming back home. They wanted to see absolutely everything.
I dont have a figure for you but our only costs was food and fuel, the whole trip was relatively inexpensive, we enjoyed roughing it in the car and living on the cheap.
Everything I have said and listed above related to my trip and i hope it helps you plan your trip. Sometimes planning can ruin a trip, we toured australia and travelled south east asia and learned that too much planning is sometimes a waste of time and can lead to disappointment. A general outline plan is best.
Best of luck to you, hope you have a great time.