Sorry but I didn’t watch out for used Strida prices 
(500 Pound for both may be Ok - if they are in good condition.)
As said; I’d not buy an older one than 5…maybe you got me wrong before:
Imagine you buy the version 3 and the rear beltwheel is worn out.
The belt will start jumping and the bike will not be rideable.
You CAN NOT buy replacement parts - worst case is that you have to throw the bike away.
Did you ever ride a Strida bike before btw?