Do I need a ring and pinion installation kit for my gear install?

its good insurance since you will be in there anyway. i had about 27000 miles on my car and replaced the bearings as well. i used the same kit you linked, the M-4210-B.

if you have less miles, you may be ok, but for an extra $100, its worth the insurance.
 
Best to do it all and be sure, did it last weekend myself.

When you get down to it, you just wouldnt feel right putting the old pts. back in...

Piece of mind... "hey, whats that noise, is it my gear or is that my old ...?" won't happen.

JMO