My 1986 Mustang GT?
Glad to hear you're ready to start modifying your mustang. There are so many routes you can take, it would be way too much to list them all. Overhaul can mean so many things, however, to keep things fairly simple I would look to purchase the best "packages" you can find. This will help to take the guesswork out of what to buy and ensure you get all the parts you need for a particular job.
For example, rather than rebuilding your motor yourself and having to worry about little things like pushrod length and compression ratios, you could save yourself the trouble by using an already built crate motor from companies such as Ford Performance Solutions or Roush Performance.
For suspension upgrades, I would look into one of the “suspension paks” from Steeda. They have three stages to choose from. Maximum Motorsports and Griggs Racing also offer very comprehensive suspension packages.
If you’d also like to clean up your interior, check with a place like Latemodel Restoration Supply. They sell all sorts of restoration items such as seat covers, carpets, floor mats, door handles, weather-stripping and more.
Beyond that, I would just pick up a few Mustang magazines and talk to a few people at local car shows. Its always best to see what people are doing in person and its always fun to chat with fellow enthusiasts.
Good luck and have fun!