Hi there,
I did the Rally in 06 and made it in a 1991 £50 K10 off ebay.
It was amazing and we went through turkey, georgia, azerbaijan, iran and all the stans before Russia and Mongolia.
The car was simply incredible. It was completely standard, no mechanical modifications whatsoever. All we did was take out the back seats, put a roof rack on and put on four new tyres. One of the tyres even had hedgehog spines in all the way from Turkey to Mongolia without deflating!!!
My advice would be to leave the car as it is. If a local mechanic wouldnt mind sponsoring you and will be willing to chuck in some free parts or change tyres etc then go for it, but the rally is really about the adventure of dealing with what goes wrong.
In Mongolia, all 4 suspension coils snapped in one day, we drove 9 hrs without suspension on potholed dirt 'tracks'. At the next town we negotiated for two days with hand signals to get it put on the back of a carpet truck, with us in it, all covered up with carpets (we couldn't see out). We were then smuggled (they weren't permitted to carry anything other than the carpet cargo) to the capital which took another three days sat in blackness!
If you are desperate to do something to the car then I would suggest better shock absorbers but please resist the temptation to mess around with it too much or take many spare parts, things WILL break and they will be parts you won't have carried as spares anyway. In fact, I saw many people who beefed up their shocks only to have them all snap thanks to the utter ####eness of these central asian roads. You will be passing through countries with some of the most amazing bush mechanics who will bodge ANYTHING right again for the price of a bag of crisps. I wouldnt bother with the whole race mentality, blaze out of Europe then just drive carefully, dont smack too many potholes, enjoy the scenery, get ####ed with mental locals and enjoy the adventure.
TOP TIP: Bring a pillow, ear plugs and tunes and some emergency food.