Buying a car in Germany and travel with it around EU

Hello experts!

I'm holding a Romanian citizenship and currently living in Bulgaria. I registered me and my family here and all of us have Bulgarian ID's.

Our plans are to buy a car and a trailer caravan and go for a year or-so around the continent.

Looking here in Bulgaria I understand that the condition of most used cars are not that awesome, and thinking about repairs while abroad makes me think it'll be better to get the car and trailer in Germany and bring them here.

After finishing the trip, we will probably not come back to Bulgaria but rather stay in Portugal for a while.

So...

1. Considering we're going to travel for a long period, should I register the car and trailer in Bulgaria? Do I Have to do it?

2. How do I register and insure the car in Germany? can I do it with my Romanian passport?

3. Are there any more things I should consider with this scenario?

4. Who are the professionals who offer services escorting clueless people like me in the whole process of selecting the right deal and helping with the bureaucracy? Can anyone recommend one?

Thanks a lot!

Most likely you will have to do 1.

You can register car and trailer in Germany, but _you_ must be registered yourself (have a valid address of living) in Germany to do so.   
And, besides, there is a good chance, that registration in D is much more expensive, because of insurance and  car tax to pay.

I dunno what you have to do for the registration in BG; best would be, in case you could take the docs for car and trailer from D to BG, get the license plates and then attach them to the vehicles. Depending upon season, cheap flights between D and Sofia.

You can also get special license plates in D, for export of car/trailer (to BG), valid for a few days only.

Or, may be you have some contacts to residents in D, they might buy, bring car/trailer to BG and sell to you. Customs, if any, should be possible to be done within BG in this case.

So, various options.