Most manufacturers recommend shock/strut replacement at 50,000 miles, depending on your driving style. However, I am a big believer in not fixing things that aren't broken. If your shocks and struts aren't rusted through, leaking, or letting you shake all over the road, you don't really have to replace them. It's a good idea to keep an eye on them and check them every other oil change or so, though.
If this is your first project--or one of your first--it's a good idea to have a shop do the install. For one thing, it's pretty difficult to do the shocks if you don't have a lift, air tools, and a strut spring compressor. For another, changing out the struts requires an alignment, which in turn requires a shop. A good shop usually charges about an hour per side, depending on their labor rates. Your best bet is to call your local shops and see which one has a mechanic that's familiar with Mustangs or Ford vehicles and ask what they'd charge you for labor and an alignment. You'll also want to see whether or not they'll let you supply the parts, or if they insist on providing their own parts (this isn't usually an issue, but some shops are picky about what parts they'll install).