Vince.
If you wait till you can afford to do it all at once you'll still be inexperienced.
While you're waiting for that far off day to come you should get yourself a 67 or 68 coupe to drive around in. They sell reasonably cheap on ebay. You'll become familiar with the different aspects of maintaining a Mustang and how it's put together. Then when you see one thats done up nice you can compared it to your stocker. When the time comes to buy your fastback you'll know a lot more about Mustangs and what works best.
You should also subscribe to a Mustang magazine and join the web site I recommended above. You might even become a Mustang expert.
AL
BTW I have a 67 coupe for sale. It's a 6 cyl std. and needs floorboards. $3000