Ok, my $0.02. I bought and sold several *fish(Sunfish and Minifish) along with a plethora of knock offs.
1) Sails and rigging are important, but only for figuring out your final offer. They are cheap to replace(if you are not officially racing, at least): http://www.intensitysails.com/rastprsafors.html
2) Hull - check for blisters, breaks and soft spots. Sunfish are light, so there will always be some flexing, but you should not hear fibers breaking with moderate force. If it has a port, open it and check for water logged foam and/or evidence that there has been a substantial water intrusion. If there isn't a port on it now, make a note to put one in. A dry Sunfish should be fairly light, with no sound of water sloshing when lifting the bow. Scratches in the finish are just cosmetic unless they make it into fiber. Just mix up some epoxy putty, fill sand and sail. Paint the hull if the color bothers you
3) Make sure the rudder hardware is all there and not bent to hell and back.
4) Check the rudder and daggerboard for rot. Budget to replace if they are(unless you REALLY like epoxy repairs).
I once bought a Sunfish that was stored outside for $200 with a trailer. Rudder was amazingly functional. Daggerboard was rotted at the top, but I ended up cutting out the soft wood and filling it all in with West Systems epoxy. It ended up being a perfect leeboard for a homebuilt sailboat I made for my sons. Sail and rigging were toast, but I picked up the sail I linked to above, and it was great.
Total cost, maybe $350, and my kids and I sailed the hell out of that boat. Now, that being said, that was in Kansas, where you don't have boat titles. So, I could pick up boats from backyards and whatnot for next to nothing without worrying about titles. So, good luck