The key item to avoid in an older boat is delamination - soft spots. If you can make the deck / hull noticeably deflect by pushing on it with the palm of your hand, then just walk away. It's not worth it.
Is an '86 the same boat they make today? Well, yes and no. The hulls are the same shape; the big parts are mostly the same. However, there have been many small changes over the years. Individually, they are insignificant, however, there have been so many that the cumulative effect is significant. Many of these changes don't affect performance, but significantly improve the boat's reliability and durability.
How much is an '86 worth? It depends - was it stored inside? Or left on a beach? Does it come with a trailer? What shape are the sails in? The trampoline? Sailed in salt water or fresh? You can see there are a lot of variables. Most '86s in average shape will go for ~$1,500 with a trailer.
Will an '86 be competitive? Again it depends, but probably not without a lot of $ put into it. Biggest item will be new sails, and maybe new rudders. Those items will cost more than what you spent on the original boat. You'll need to glue the frame together to get it stiff enough. Even so, those will only get you to the middle of the fleet in the most competitive events. To get to the top end, you'll need a relatively new boat (less than 5 years old).
I don't want to discourage you from doing this - on the contrary, an '86 in decent condition is a good "starter" boat for getting into racing. It will have a Comptip, rake-adjustable rudder castings and low-profile mainsheet blocks - key requirements for racing. You'll be able to learn how to start and get around a race course - honing technique and sailing skills. Once you've got that down, you can move into a newer boat and take your speed up another notch.