When looking at Beetles your number one concern is always body and by body your big problem is rust. Depending on where you live, the run of the mill Beetle in your area may be anywhere from rust free ( think the Southwestern US ) to rolling iron oxide as in up here in Southern Ontario. Look especially at the floors, the heater channels ( the thresholds of the car's doors ) and on a Super Beetle ( as your ' 77 is most likely ) the strut towers under the front luggage compartment cover. The areas around the lower door hinges are also prone to expensive rust repairs. As for what it is worth, shop around by viewing Beetles on Collector Car Trader.com and E-bay. This will give you a fair idea what other ones in various states are selling for. Look around for a club in your area where you can hook up with someone who knows the cars. This is a good start for the process.
I'd rather have a partial bottle in front of me than a partial frontal lobotomy.