Hello and wellcome elefantman!
You change the original size and you perhaps -I don,t known this laws in England- can have problems in the "vehicles technical inspeccion".
"The fact that 16" rear wheel was choosing , apart from for the good behavior in march, for the following advantages: a minor weight, a minor solicitation of the driving chain, minor height of the seat and distance between axes
Opposite to these advantages, the disadvantages of the small wheel (scanty vertical clearance up to the soil, the major facility with which the irregularities of the road surface are transmitted to the frame) were considered to be slightly relevant."
A traslation (thank you Pedro) from DDR magazine “Kraftfahrzeugtechnik” (Vehicles Tecnologie) about ETZ 251 model but applicable also TS 250/1: front 18" "... with the front wheel 16" the fork big sinks when use brake to stop" and rear 16".
Rear suspension "Hagon" for TS, photo from the Lamaha post:
download/file.php?id=1351For tires my little experience: Dunlop K-82. Excelent in all weathers. Good grip, K82 I see in Spanish classic races,it is less expensive that Avon.
I think "classic carved" tires better than "modern" to old MZs, well, perhaps is a subjetive opinion.
Saludos, Manuel.