You'd probably be better off to look for the best boat you can get for your budget rather than the largest. Starting off in the 28-30 foot range is going to give you a much wider range of boats (in better condition) to look at.
In the end you'll get a boat you can use right away.. the costs involved in 'great deals that need a little work' tend to escalate beyond what you'd have likely paid for the same boat in good condition.
Here's a link to a worldwide search for boats 28-32 feet under $20K (some results beat the price filter, but still there's plenty there and you'll get some idea of what's available in that range.)
(Sail) Cruiser/Racer Boats For Sale