First, consider what design of wetsuit you want. As a (casually) competitive kayaker I use a 2mm longjohn wetsuit in winter with a thermal rash vest (or two if it's really cold) and a waterproof if it's very windy. Sometimes that's too warm for a proper training day, even in the depths of (UK) winter.
For casual, whitewater or touring use I wear an extra thermal layer under the wet suit along with full waterproofs. If you're not planning to swim then you're not going to get too cold.
For sailing you'll want to be warmer, you spend a lot more time sitting around doing not a lot in the wind, but I'd still suggest longjohn as it keeps your arms free. Wear with thermals, jumper, waterproofs.
A full dry cag (latex rubber seals, double layer neoprene waist for spraydeck interlock) is going to be very hot in the long run, I wouldn't suggest it unless you're expecting to swim. Semi-dry cag (neoprene seals, elastic waist) with ordinary waterproof trousers should be about right.
Either way, a 7mm full wetsuit will completely immobilise you for the sake of sailing or kayaking. Guidance for diving or surfing isn't suitable for kayaking or sailing as they spend a lot more time in the water. If you swim when you're sailing something has gone wrong.
On a used wetsuit, check seams, wear on seat and knees, zips, velcro.
Unless it's really cheap, I'd ignore anything that's been repaired, wetsuits aren't that expensive new. However: Repairs could be patched or glued. the neoprene glue holds but doesn't stretch well, this will be the probable cause of any repair failure in the long run.