As to whether or not you should have a limited slip differential even though you are 55 and will "never" race or drive fast (sure), it is a very good thing to have, especially on a relatively high power car. The traction control actually "fights" your inputs, and can, in my opinion, be quite dangerous, seriously reducing performance if you get a bit of wheelspin when pulling out into fast-moving traffic, making you a broadside target instead of an object smoothly accelerating into the stream of traffic, or really messing up your control of the car in a corner (where you should be applyig throttle). I have it on my '97 Z-28, and it scared the daylights out of me several times in critical situations. It is on my '06 GT (stick), and the traction control button is pushed and the off light checked immediately after engine start as a matter of course.
Anyway, as to the limited slip differential, if you ever get into a muddy area or the car gets stuck otherwise, or hit a slick spot on the road, especially on a curve, you'll be very thankful that you have it.
BTW, being 55 doesn't mean you have to drive in the official AARP approved manner - I'm 54, and, while I don't race anymore (not much, or seriously, anyway), I always drive in a rather "spirited" manner, it's much more fun than "fuddy-duddying" around, especially in a car with a good suspension, good brakes, a smooth-shifting 5-speed and an engine that has good power and loves to rev. My Dad raced sport cars in the '50s and early 60's, starting with an MG TD and progressing to a distributor/factory-backed early E-type Jaguar ride, including at Elkhart Lake before the race track, when they closed off city streets and county highways to form a road course for the races. He used racing technique in daily driving, and taught me the same. I raced big-bore sport cars in the late '60s and '70s, primarily SCCA A-Production with 427 Cobras and 427 Corvettes, and I use racing tecnhique on the street, also - gives you much better control and is a lot safer then the "point the car and push the gas pedal" method used by 99% of the "drivers" out there. Dad never had an accident, and I was the victim of a careless driver only once (head-on, she lost control of her car in a deep puddle about 20 feet ahead of me, I tried to avoid her, but it was just physically impossible to miss her, although I did manage to change the point of impact from nearly direct on to the left front corner of my car). Remember, the point of the Mustang is to have fun! You don't have to race it, but it is fun, under safe circumstances, to let the cat out of the cage a bit every so often.
Drive Safe and have fun!