Since no one else has provided any words of wisdom, I will try to answer some of your questions. There was hard luggage made specifically for the MZ range similar in style and construction to the Krauser luggage used on BMWs. I have in the past had bikes fitted with this luggage. Personally I never used as I several bikes and prefer the flexibility of soft luggage. Sadly from your point of view these bikes are long gone complete with their luggage. I would suggest keeping an eye on eBay, (German & UK), and putting ad ad in MZ Rider and on the classified section of this site. Craven panniers are like gold dust these days and fetch a high price as all the Brit enthusiasts want them as well.
Regarding converting dual seats to single seats, this is easy enough to do. I have previously converted a TS250 seat and a TS150 seat. The only slight complication with the ETZ seat is that it just has sliding clips at the front rather than bolts so you have to invent a way to secure the base to the frame. If its of any interest I have a framework which was made up by Dennis Poulter to give a single seat and a carrier section and should work on an ETZ. You are welcome to collect it if you don’t live to far away; I doubt it worth the cost of posting. My contact details are in MZ Rider (Membership Co-Ord) if you want more info.