Hi Carole, I'll go through each bit at a time.
I'm sure that insurance will be OK. In the end I got my insurer Credit Agricole (so not much use to you) to issue a European Insurance Document which would cover everywhere within the EU. Now it would also cover Russia. As for beyond, well a look around on here usually reveals that you can buy insurance at the border. While they might scratch their heads a bit, maybe the paying of a small additional "fee", might help.
I can't say I'm an expert in quads, we were looking at off-road buggies and side-by-side vehicles, but I picked up a bit of an idea hanging round too many fora. The HUGE problem with quads is their truly appalling fuel consumption, some of them will only average 10-15 mpg. If you think you might be doing 12,000 - 15,000 miles that's more than 1000 gallons of fuel. In places petrol stations are few and far between, so you'll have to carry plenty of spare, which will eat into your luggage space, add loads of weight to pull out if you get stuck, and no doubt lead to more potential breakages.
There are a few diesels though and although they're possibly a bit slower, it's all about taking in the sights, isn't it?
Roush do a converted Cam-Am, which might be interesting, but probably expensive.
Arctic Cat do their Super Duty Diesel, both of which are FAR more economical than petrol and both are good brands. Sadly neither has a lockable rear-differential, so your tyre wear will go up, and of course you'll have tail out fun on wet roundabouts. Still they're both well-known brands so spares (including tyres) should be easier to find.
This forum is littered with threads detailing the Chinese problem. Basically, you can't drive in their country without Chinese registration and a guide, and that's VERY expensive.
Actually we had several, they were all devised to avoid Ewan & Charley. I favoured going via Scandinavia, ferry from the UK to Norway then up across the top of Sweden, Finland and into Russia stopping at St. Petersburg, Moscow and then across, down to the Altai and into Mongolia that way. I just thought many people take a more southerly route, and I love Scandinavia.
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The most important bit seemed to be what time of year do you want to be in Mongolia? People have driven the "Trans-Siberian Highway" in winter and said it's OK, but Mongolia in -40° conditions, no thanks. So the idea was to get there around June/July and get out and down to China by September/October
Of course, and that will add even more to your fun.
Merci à toi aussi (but really I'm an ex-pat Brit.
Lack of money and time, and the thought of wrestling with the Chinese. Oh and being lured away by the allures of young ladies on motorbikes (well one anyway) and having my head turned towards Africa. I will do it one day though, but probably by bike or slightly ill-suited soft-roader.
Alex