First I’d download the manual; at the pages “center” (following page 11) you can find the exploded drawing of the Strida 5.
https://drive.google.com/file/d/0BzaK6supp6fLMm5CM1ZPN2txNEU/view
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Check if there are any cracks visible at the lugs of the frame around the bolts part nr. 274, next to the locking latch nr. 216.
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Check also if both bearings 100-03 and 100-04 are intact.
(They do not need to spin perfectly - much more important is their completeness.) -
Take a closer look at the “ears” of the frame, next to the handlebar release buttons (p.nr. 215-14-2) - there should be no sign of wear.
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Close the steering clamp lever 215-06 completely and check if the steering bars wiggle.
(Wiggling bars wear out the clamp if not tightened in time.) -
Look very close on the top of the freewheels (340-05) teeth:
If you can see a small rectangle on top of each tooth will it be a new one.
No rectangle, instead of just a round edge means wear.
How “free” should the freewheel be? -
Abnormal play of the bottom tube can be checked very easy like shown here:
https://www.youtube.com/watch?reload=9&v=bdECyvRsizI
Pay attention to the belt tension - it changes dramatically while straining/relieving the left pedal! (On this bike was the lower bottom tube bearing 100-04 defective.)
If you ride one or the other;
you may find it unstable…
That’s, for my meaning, just partially “because it’s a Strida”,
please note that the LT plastic wheels are one of the in sum just two(!) reasons which make the LT series cheaper, compared to all other Strida bikes.
You may want to upgrade later also to sturdier, metal spoked wheels I believe.
Pardon me, but I’m not sure to have understood your last sentence completely. 
You mean the efneo three speed I guess?