For auto-travel Moscow-Varna, you will need international standards, documents and a green card for the car, visas. If you have a valid Bulgarian visa in your passport, you will not need to transit through Romania.

From Moscow you can travel along the Minsk Highway to the border with Belarus and further, through Kobrin, to the border with Ukraine. From there, through Kovel, Ternopil, Chernivtsi to the Ukrainian-Romanian border. Here you need to buy a vinette - a ticket for travel on the roads of the country. Then the route will pass through Suceava and Buzau to Bucharest, and following the signs for Giurgiu (it's Gyurgevo) to the automobile bridge of Friendship - the border with Bulgaria. Buy a Bulgarian vinette and through Ruse, Razgrad, Shumen, Kaspichan, Devnyu to the goal - Varna.

You can do without arriving in Belarus, if you go from Moscow via Bryansk, go through the border with Ukraine and follow the route Kiev-Odessa-Bolgrad. Afterwards there will be a border with Moldova, payment of the toll, a way through Vulcanesti-Cahul, then the border with Romania. Then Galat-Braila-Kalarashi, from there by ferry to the Bulgarian Silistra and on the highway to Varna. Either after Braila through Slobozia, get to the Danube Bridge (Friendship) and then - as in the previous version.