First off, you don't say whether this boat has a mast or not. Does it have a mast, boom, standing rigging to support said mast, running rigging, sails, an engine or outboard? If the answer to any one of those questions is NO, then I would say, you'd be better off looking for a different boat. $100 is a gift to the guy selling the boat, unless it is basically complete.
Getting a mast, boom, sails and rigging are going to cost you more than getting a boat in decent shape will. That doesn't even figure the cost of replacing and re-installing the rudder—assuming no drastic damage occurred when the rudder was lost.
Generally, as a rule, it is cheaper to buy a boat in ready-to-sail condition, than it is to get the same boat in not-ready-to-sail condition and refurbish it to the same condition as the first boat.
BTW, your basic description is so generic and lousy that it could describe any one of a thousand boats. Some of those boats aren't worth spending the time and effort to restore, others are... without photos, your description is basically USELESS.
I'd highly recommend you read the POST
in my signature to help you get the most out of your time here. It has tips on searching sailnet, writing a good post, etc..
If you're really set on going ahead and buying the POS you describe in your OP... then you really should read the Boat Inspection Trip Tips
thread I started.