Well I definitely came to the right place to ask for advice - thanks to all respondents, I will heed your advice.
I should now have enough knowledge and confidence to ask the right questions and examine more closely.
I already feel quite weary about this sale, since it has so many non-original replacement parts. Perhaps a lower price will need to be negotiated, provided rail alignment repair and drive chains/sprockets are in good condition..

When checking drive chain for serviceability; am I checking for stretching (slop on sprocket) and tension?.
Sprockets serviceability; checking amount of teeth material? ie. not sharp/pointed or wave shaped.

Thanks again for your knowledge, much appreciated.