The OHV outfits are 650cc and geared for around 55mph.
I am sure an OHV solo would go well beyond this on solo gearing, but I've not ridden one to say precisely.
The sidevalves are 750, but suffer the usual sidevalve issues of heat and inability to rev. My solo is good for around 65mph absolutely flat out - if you ride it up to that for any time then it will overheat. Alot. Happiest around 55mph.
All are boxer engines so lovely and smooth - very little vibration.
The sidevalve has more torque as it's a larger displacement. It is also quieter with less parts to maintain.
I think it has more character and isn't really disadvantaged when hitched to a chair - the gearing limits both the OHV and sidevalve as the shaft drive gearing options are limited.
You can argue the merits of the dnepr 650 OHV and the Ural 650 OHV.
Later Dneprs suffer from alot of quality issues, as the factory was struggling before and after the dissolution of the eastern bloc. If it's been owned by someone who has been using it, then it's likely to be sorted though. I think the dnepr factory struggled on into the late 1990's but still produced the occasional parts and bikes after that (opinion seems divided) but it's effectively been end of life for around 15-20 years.
Ural is still going BUT has been making their 750 OHV's, mainly for the american market, since the early 2000's. These are a different engine and the factory I don't think makes any more 650 stuff.
However, there are parts a plenty given the huge production run of likely millions of bikes over the years and the number of owners still running them makes selling things viable.
The CJ750's are effectively the russian M72 and ther are later upgrades to a 12v engine with starter. CJ750 and M72 are largely interchangeable and mostly interchangeable with the K750 engine (K750 uses a swingarm frame)
Like most old bikes, the prices of russian stuff has crept up over the years. Especially the older stuff with plunger frames
Very few are likely to be original as parts are fairly interchangeable over something like 50 years. Thus my ostensibly early 60's outfit has later "dnepr leading links and front TLS, I have a slightly later engine (with filter).
If you are asking me for a recommendation .. buy a sidevalve 750.
If you can get one then go for a Cj750 or M72. The plunger frame makes them much lower and easier to manage.
Get a later TLS front brake, replace the original carbs (if russian) with CJ750 ones and put an electronic regulator on.