Andy, I am somewhat curious by your statement of no 4 X 4 driving.
Most of Mongolia is basically off road, there are very few roads as most everything is dirt tracks etc. This means driving through rivers, through bogs, over mountains, etc. The only finished road that I know is from Ulan Bator north to Siberia. There are some roads being built, but many times you end up leaving the road and back onto the dirt stuff.
Most of the time you can make it without being in 4 wheel drive, but it is nice to have.......I would at least have a winch on the front of your vehicle. Most Mongolians will be happy to tow you out of tricky stuff.
Mongolia is a fantastic country with amazing sights away from the capital.
But don't go in the winter.....if you break down in the countryside when it is -50 Centigrade it could be the end.....so be smart.
Have a wonderful trip.