If you want FAST .... Id suggest an Ultimate 20 (PHRF @ 145 or Portsmouth Yardstick rating @ 79.9), quite quick and with a large cuddy cabin. U20 Class Association
. Used prices will be $15K+
More in your $5K budget range: Another with a small cuddy cabin would be the Holder 20. ( .... at about Portsmouth 84-85 rating, or PHRF 175). http://www.holder20.com
Comparing the U20, the San Juan 21 would be at about 260 PHRF or 95 Portsmouth ... (a pig! if you're looking for 'fast'). The only problem with a 'fast' boat such as the U20 is you will dominate most PHRF/Portmouth fleets and won't be 'progressing' with your sailing skills as fast as with a 'mid fleet' rated boat. Being well in front of any mixed fleet all the time gets boring real fast.
FWIW - racing Portsmouth Yardstick handicap is VASTLY superior to the stodgy and political PHRF and you might want to consider to locate fleets that race under the Portsmouth Yardstick rules. The reason is with PHRF and for FAST boats of near 20ft. LOA, many times you're beaten by the PHRF 'numbers' before you even leave the dock. Portsmouth ratings corrects for varying wind strengths, etc. Portsmouth Yardstick Index