Can't agree with farqhuar's comment about a 4x4 being a magnet for thieves at all.
We did Germany to the Far East for 1.5 years returning last year and we didn't have any trouble with thieves at all. Far from it. In fact we found the Central Asian people to be the most friendly and warmly welcoming people we've come across. Often their first question is whether you are having a nice time in their country and they want to ensure you leave with happy memories.
The only time our land cruiser was ever touched was by the cows scratching their backs on our spare wheel at the back!

Robert's and Bermuda rover's comments I can relate too, but you can get to song kol without the 4x4. You can even do Mongolia without, but you really would miss a lot!

The lada will be too small for you, but they are great.
The delivery van is a great, potentially much cheaper option and we travelled with some sling our route: basic but functional. For size you might prefer a transporter though. The Mercedes are the local buses in the central Stans so getting spares and repairs is not difficult at all.

Pics of fellow travellers on our blog: we had everything from cyclists, suzukis, 35 year old Mercedes and totally 'normal' white-box caravans.
www.wanderwheels.wordpress.com

Have fun on your trip!