Here you can pick up a daily driver 85-86 V8 / GT convertible for 1500-2500 pretty easy. I passed on a 86 GT that just had oxidized/faded paint and a lightly worn (dirty looking) interior for $2000 from a guy at work just a couple months ago. I was considering it for my daily driver. I drove it and it ran great. It took him more than a month to get his money for it. I'll bet their value isn't much higher there.
The early 80's models do not have much resale value. I find the value in my '83 convertible to be in the fun driving it. If you are buying as an investment to make money, it is not a good deal. If you are buying it for fun - go for it! It is hard to beat driving a Mustang convertible (any Mustang convertible) with the top down. My SWMBO loves to get out in the evening when the air is a little cooler here in Houston and enjoy the stars and sound of the night going by.
Personally I believe that pricing on the pre-fuel injection V8 convertibles will start heading up soon. Just my opinion, but I would keep my car anyway.
Grab it, I would. How can you go wrong for that price? True, you will have to swap k-frames to get a V8 in there. But why replace the top? Put it down and leave it down! Just don't drive it in the rain. These fox chassis mustangs are so easy to modify, its almost, ALMOST, a joke.
At the price I say go for it! I have 3 fox body verts and all are cool!
I hope your right about values going up as I have less than 1700.00 in my 3!
83 GLX 5.0 is a bit of a beast due to upgrades!
84 turbo GT is still being put together but its cool!
91 2.3 auto for the neice to drive while she wishes for dad's 70 vert.