If the boat is in good shape, buy it. Nice used 22s as other C-Dory boats are hard to come buy. Then, go use the 22 as powered. It takes about 20 minutes (some have said 10 but I always dropped the pin...could not find the prop wrench, the grease tube rolls down the driveway... you get the idea) and never could change out and store the removed prop in less than about 20 minutes.... One prop for 2 folks on board and not much stuff.....and then another one for 4 folks and stuff....or simply a lot of stuff for a long cruise. if you like to "Trawler Cruise"...at say 6 to 10 knots and enjoy all the sites of close water boating.... then you may find the 50 you have sufficient, cost less, and operates more echonomical. I have to say that I truly have logged many more hours going 10 knots or ....much slower than over 15 knots..... Then....if you do not like the power set up, sell it and go for the 90 (same weight as the 70)....or twins.. See other threads on endless discussion on twin verses single or single with kicker. Your boat, your needs, your money, your style and type/areas of use,....
But, buy it, use it and enjoy!
Byrdman