Building a 50' boat would take over 10,000 hours and over $100,000. A used seaworthy
boat would probably cost more and possibly require less time. You do the math.
Your question could be "Where can I find the time and money
I would suggest that you do things the other way round. Work out what you want to do then decide on the best way to do it.
The best and cheapest boat is no boat.