I've rebuilt several alternators myself, including the one for my 911.
IMO, it's really easy to do, not sure I'd ever pay a shop to do it unless I couldn't source the parts myself. (Some of the parts, like the rectifier, can be difficult to purchase retail -- at least for these alternators)
On mine, I replaced the worn brushes and bearings, then polished up the commutator with a bit of sandpaper. My car is similar mileage to yours and the commutator still looked to have plenty of life in it.
Of course, this isn't a ground-up rebuild, but I find that those are rarely necessary, unless you're dealing with voltage issues from the alt. And if that's the case, it's usually an issue of a bad rectifier (low voltage, excess AC voltage in the system) or voltage regulator (high voltage or sporadic charging).
To replace the brushes, you can either solder new ones into the VR (easy, basic soldering), or get a whole new VR unit with fresh brushes (VR replacement is super-easy). I got the brushes for like $7 on ebay. (dimensions are 4.5mm T x 6.5mm W x 14mm L x 46mm). Replacing just the brushes saves money and means you keep using a known working VR.
I used a shop press to drive out the front bearing on my alternator, but I bet it's possible using a socket and hammer. I was able to get new bearings from McMaster by just entering the codes on the old bearings, cost me less than $20 for both.
If you do it yourself, be careful with the little plastic cup that holds the rear bearing in.
If your alternator is putting out correct voltage, I wouldn't worry about replacing the rectifier. And at that mileage I'd be surprised if the commutator is worn beyond use.
Honestly, I think the hardest part is just getting the alternator out of the car itself. John Walker had a great tip about separating the alt from the fan housing. (
http://forums.pelicanparts.com/porsche-911-technical-forum/454747-can-not-seperate-alternator-fan-housing.html)
So, yeah, taking it to a rebuild shop is probably the easiest, but I'd say that rebuilding it yourself is pretty low-difficulty if you're fairly handy.