My knowledge of 150s is nil but I own a 1051 which is essentially the same construction.
A mid time engine and prop should last you 10-20 years before overhaul as long as you use it regularly. The performance may decrease slowly over time but not dramatically. Expect what the book says.
Wood glue deteriorates over the years. Modern glue seem to last forever. It is likely that the fabric has not lasted 50 years. it will be recorded in the log book when it was last changed. At the fabric change the wooden structure should have been inspected. Again this will be recorded in the aircraft's log book.
I was told to look for rot in the structure when I bought mine. Gravity sends moisture to the low points on the aircraft...elevators, tail, aft of the wings and the aft longerons. Gently pinch the wood with your fingers. Softness indicates rot. There are wee drainage holes in the underside that need to be kept clear of mud and debris.
Properly looked after wood and fabric will outlive you but a dry hangar is essential.
My engine is not an O-200 but they are extremely good reliable engines. The rules for experimentals/homebuilts are nowhere near as strict as for certified aircraft. You need to talk to these people to ask the same questions:
EAS Experimental Aviation of Switzerland
Experimental Aviation
Look after the tailwheel.
Because I don't know the rules in Switzerland I cannot comment on the cost of an STC. I suspect there is not one, the EAS have sorted that for all homebuilders.
Finally all Jodels fly lovely and the Mascaret (which I have never flown) has the reputation as being the best.
Do your homework and talk direct to the homebuilders association in Switzerland