You might try Gumtree, and report here what you see.
Perhaps you'll get good advice on a good deal. On top of bc's great prices of course.
Perhaps go to the local spot and talk to local sailors - they might let you try a bit (I have for keen people like you).
155 seems a bit big, but all depends: what is your weight, how did you fare, are you otherwise sporty? Do you intend to put weight on

Is it going to be for flatwater mostly the first year?
Are you aiming at strong winds only, or willing to go out in lesser conditions to improve quickly?
How many years do you envision your first purchase to last?
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