Hello Zippy,
It all depends ...
Basically on how the car looks, what condition it really is in and finally the price. Especially the second is not easy to judge for a newbie. Good looks does not automatically mean that the car _is_ really good ...
Please look & read the buyer's advice here on the group's homepage first.
Eventually, you should put about 2-3000 $$ down of the usual price for a given condition if the softtop is missing. If your car is the California coupé (with rear seat bench but without softtop compartment) it's much less worth.
Most important is the overall condition of the car, not the missing softtop itself. If it's really mint and somehow reasonable, you should probably go for it.
Good luck!
Achim
(Magdeburg, Germany)