An Englishman moving to AL -- boy are you in for a surprise...
What you speak of is becoming more and more common these days. Many, many members of this board have made these upgrades to their Mustangs, so you'll find a lot of information on the subject here.
It really depends on how modern you want to get. Fuel-injected 5.0 pushrod engines are pretty commonplace and the entire conversion is available over the counter, plug and play. The state-of-the-art modular engines are finding their way into early Mustangs, but it's still pretty uncommon.
Suspension is a similar story. You can get really high-end replacement parts for the factory
suspension, but then they're still high-end parts for a 40+ year old design. Mustang II-type front end conversions are easy to do these days and make an incredible difference in the ride and handling of an early Mustang. Not so many truly modern upgrades are available for the rear
suspension. I'll probably get jumped all over for this, but I don't consider a solid axle on leaf springs to be a great riding or handling system no matter how good the parts bolted to it are.
I'll get flamed if I plug my business too much, but check out my site at
www.reenmachine.com for more info on this specific type of car.
Have fun!
-Pete