You can get the gear installation kit for cheaper than that from www.stangstore.com
The installation kit isnt much more than the little associated part of the differential. Pinion nut, crush sleeve, some shims, and a few other little things. When doing a gear change, be sure to use some friction modifier.
If that's for everything, including the gears, I say go for it. Of course, the lowest price I found excluding the gears here in Utah was ~$500. Had to drive up to Idaho to get it done for $200.
You need the shims, and since you have it torn apart, why wouldn't you want to replace the bearings? I guess it would depend on how many miles you have on it.
Well, do you already have the gears? Did they come with shims and a new crush sleeve? You might get by without the shims, but you must have a new crush sleeve. If I remember right, I picked up my gears w/install kit and new bearings from GEFRacing.com. You may not need new bearings, depending on your mileage. However, during the course of my install, we sorta broke some of the existing bearings. Oops. I was glad to have the kit.
Just chat with both shops to see what they recommend. Personally, I think it's better to have it just in case. But that's just my experience.
I bought the FRRP gears and they came with everything needed for my install (crush sleeves etc). If you have low miles on the car you should be in the same situation if you purchased Ford racing gears.