This is only from my personal exp
Things to make sure you have
Log Book
if not your name on the log book a written reciept
you will need your residency card
Bulgarian insurance which you can do prior to registration as it can be done on the vin number when you have bg plates you can inform them of the reg number
do take with you an official transator to fill out all your forms,it will be money well spent and cost us 10 Lev for half a day
most forms that you get you will have to pay for unlike the uk where all forms are free,here in Bg you pay for all forms you get,although only a few lev so no big deal.
you must also make sure to get vinettge (road Tax) when all this is done you then have 2 weeks to register your car for the municipality tax,when you register your car it will also be moted for 12 months.also make sure you have the stickers on your headlights or it will fail.when all the paperwork is done mot then you will be issued with your number plates,then your all finished,just one extra thing take a screwdriver to take your old english number plates off as they will ask for them,and they like them clean,i think thats it if i think of anything i have missed i will add it
Mike