1. How much space, either under cover or in the open, have you got at home?
You can get a classic racing dinghy (well, most classic racing dinghies) in a garage - a dayboat or bigger is a different scale of problem and working outside slows you down a lot.
2. What workshop facilities have you got?
Not that much is needed but a good bench and the usual tools are handy.
3. Neighbours not a problem?
Noise from power tools and from rivetting up clench nails caan be an issue.
4. What material suits you best?
I like wood but can't get on with GRP and metal is beyond my competence. You may differ...
5. Trailer?
I'd suggest trying a classic racing dinghy first. They are a small scale project requiring careful work but with small pieces and they often come with a trailer and can be easy to sell afterwards if you want something bigger.
There are an agreeable bunch of enthusiasts here and no shortage of good projects and advice!