I believe a 3g device can use either wi-fi or 3g
A wifi device will not be able to use 3G
This is a in the air data connection, similar I guess to mobile network coverage, smart phones use this data!. You would usually have to buy a data allowance from a mobile network and pay monthly. I would guess the coverage would be very poor in most Bulgarian villages.
will require an internet connection to your home and a box in your home to transmit the signal. The range of these is quite short although you might be able to pick up internet from a close neighbour, most would be security protected however.
So unless your willing to pay for internet and a wifi router in your home then you will not be able to use the wi-fi kindle. You might I guess be able to use free wifi available commercially in some restaurants to download the book for reading at home. maybe you can transfer a book from the 3G kindle to the wifi one. Does it have a memory card?, or maybe bluetooth?
hope this helps a little