Why do you want it? transportation, retoration,modification or race, your dream car?
How much experience do you have with mechanical, fabrication and body work?
What size engine is in it a 4,V6 or V8.
How much rust? what does the interior look like? Are parts missing?
The Mustang II can be a great progect if you truely like the lines and potential for performance both in the turns and in the straight line BUT, it can be a nightmare if you are just looking to put it together as a daily driver. I am not saying it's not a good car but you have to know what you are getting yourself into. While some parts are available new from some sources, others are not. Plan on buying a lot of used parts from E Bay, some extremely high in price and in somewhat poor condition. Some parts are no longer available such as V8 Motor mounts and NOS ( new old stock) can exceed $400.00 IF AVAILABLE. Headersfrom Headman are $170.00 and will work but are not the best brand choice, Hookers are the best but are close to $700.00. Depending where you live, the junkyards may or may not be a good source, not 1 Mustang II in any Las Vegas junkyard.
I was luck to find mine locally in excellent shape for $ 1800.00 and althought I probably should have left it stock for resale purposes all original, I decided to modify it to my liking and car shows.
To buy or not to buy is a decision only you can make using the above criteria don't ask for advice, because you are the one that has to live with your decision. Worth? post some photos and maybe someone here can evaluate a value but again it's up to you. Here are photos of mine and a link to another Mustang II site which is very active. Hope this helps and good luck with your decision.
Mustang II Network - 1974 - 1978 Ford Mustang II