Its generally thought that its your third boat, that is most perfect for your needs.
You learn alot from the first two, incorporate these lessons in the third.
Look at lots of boats, note the good ideas, and more importantly the bad, to avoid the same mistakes!
If you haven't already, hire one for as long as possible, out of season, if you like it in the cooler/duller times, you will love it when the sun shines.
Self-fitouts, my opinion, build to R.C.D. standards, the boat will then be saleable, should life turn unexpectly, a non RCD boat less than 5 years old will be almost unsaleable.
Budgets, think of a number, double it, add 10.28%, its a starting point, works for both £'s and hours. (As others have said include cost of tools needed. Decent ones cost but last, cheap ones often cannot do the job.)
Bod