I have built many NA/NB, one NC, and a few other makes/models of live-axle rear ends. NA/NB rear ends are easy to build, but a little tricky to get into spec. A careful hand with the right tools and some patience can get the job done competently. If you go to a shop that does not specialize in Mazda, go to the pointy board garage section and print off the diff rebuild write-up to take to your technician. It is a very good instructional and includes all specs.

To swap gear ratios, you will need to buy new pinion bearings, pinion nut, and pinion crush-sleeve. You don't need new diff carrier bearings if you are planning on reusing your stock differential. If you want to upgrade your diff, you will need the bearings for that as well. Use this information for budgeting.

I can't weigh in on 5sp v. 6sp durability. I'm pretty sure that topic has been beaten to death anyway.