Coastal rowing boats.

[William_H]

420's (if we are talking about the sailing dinghy) are really small. Not suitable for full sized people.
Good for two girls or two kids.

Well I think the 420 hull or even better a 470 hull might suit OP. They should certainly have enough buoyancy for one man and gear and OK for 2.
However I would think for cruising and open water you would want a "cabin" or shelter up front that can be sealed for buoyancy and self righting and a reduction in cockpit volume to reduce water onboard in case of swamping or capsize. So yes an old larger sailing dinghy might be a good cheap start for xperimenting. olewill

 

[Romeo]

If you want to row and enjoy your rowing, get a rowing boat designed for rowing, not a sailing boat that can be rowed. Fundamentally, other than being able to float, the design characteristics to maximise sailing efficiency and pleasure are entirely different for those for rowing efficiency and pleasure.

If on the other hand all you want is auxiliary power when there is no wind, oars on a small sailing boat are a great idea.

The good thing about having smallish trailable boats is that you can have more than one of them, and select the correct tool for the job.