Hi,
You should listen to the other posts above, except if you have already really well prepared your itinerary and you are equipped with GPS, and detailed maps and with someone that speaks mongolian, and with someone that knows everything about DIY car repair..
It is not that I want to spoil your illusions, but I have taken quite often the mongol minivan public transports on long distances and everytime I have seen experienced mongol drivers losing their way (no tracks at all) or breakdown in middle of nowhere. For example, in february, it took me 30 hours to do the 700 km that separates UB to the north of Mongolia, Hovsgol... The first time I took this kind of transport, we had a fire engine and we got stuck in the snow, second experience we were out of gas, third time a stone exploded the tank and we had to find any container we could find to collect the gas because of course there were no gas station a 100 km around, etc, etc...and I don't count the flat tires.. And everytime, we have not met one single car or anyone that could have helped. The guys had to do the repair by themselves or went by foot in the countryside to try to find some nomads that perhaps have some gas... So, well, one advice, try to find a driver and that knows well the countryside. He will probably charge you with something around T300 per km.. For even more comfort, you can go to guesthouses in UB that can provide you a driver and maybe a guide that speaks other languages than mongol or russian.
Have fun. Bayartai.
Nathalie.