Sounds like an adventure.
The Turmenistan visa has always been notorious for it's difficulty to get one arranged and for its inflexibility. A transit visa is the easiest to obtain. The ferry at Turkmenbashi was highly unreliable for years (don't know the current situation) and some people where trapped in the harbor for days waiting for the delayed ferry after their transit visas had run out. Make sure you sort that one out early.

Belarus is also tricky as you can really only get a 48 hour transit visa and they seem to want to know exactly the two days you plan to be in country. I had no problem as Belarus was at the start of my route, but if you are coming west you will most likely not be on schedule by the time you get to Belarus. Also, not sure where you get the visa if you don't pick it up in your home country.

You already know about the guide issues in China if traveling with your own vehicle. That one has to be done a min of 60 days before entering China, at best through a Chinese Travel Agency. And it's not cheap. You will have to pay for or provide transport and a driver for the guide. Not sure if he is going to ride shotgun on an Enfield.

Oh, and get ready for some terrible asphalt on the road from the Romanian border through the Ukraine to Kiev. I just drove that 4 weeks ago and seriously overweight trucks have carved huge ridges into it. I saw one saloon at with a punctured oil pan at the side of the road. I didn't have any problems with my Dakar prepped Land Rover but my teeth still got a good shaking. The asphalt only gets good when the motorway joins the other one coming in from Poland about 100km wet of Kiev.
Have fun!!