Hi Anton,
welcome at Stridaforum 
I’m surprised always that there are still many of these ancient bikes alive.
Also, I like to see how much enthusiasm they generate.
Just finally; I’m truly able to understand why the enthusiastic period can be pretty short…
…I guess that is the point in time where the owners realize that it is nowadays totally impossible to find certain spare parts.
As a fan, you might know already that Mark Sanders supplied some 3D files to replace a few parts.
But, for example Strida 3 metal hubs, wheels, brake assemblies etc. can’t be substituted this way.
Aluminium and plastics don’t become better over time, some of these bikes are meanwhile over 35 years old - I do believe that this fact on its own is an important safety argument.
Personally, for continuous riding, I’d not recommend a Strida older than 5.0 - simply because I can be sure to find any required spare part on the market.
Regarding purity of the original concept, well…in my opinion, Ming cycle made the Strida in general a bit better (metal wheels, moving the freewheel back) and in some details worse (rear hinge) than the initial product. But the real concept modifications were negligible - I’d see them in two and three speed drives, 18" wheels and perhaps the QRS saddle system.
Best regards,
Chris