I open all mine up before going to the auto store to assess the condition. If the model is known to be reliable, looks good, and the parts are cheap I just replace bearings and brushes and run it again. I'd rather rebuild the one I have that is in good shape than take a chance on getting a just above reject rebuild for my good one. If there is irreparable damage in the stator, rotor, case, brush contacts, or the outrageously priced (GM) regulator has a known short life I let the rebuilders have it and hope that I get an above average rebuild in return. The quality of some rebuilds is low enough that you should expect to rebuild the rebuild. I at least open mine up and fix what they missed, like anti-seize on threads.
Bearings are on eBay and Advance Auto can next morning delivery many brush sets. Be sure to buy 16,000+ RPM bearings which is the sort of thing you learn after doing it wrong a few times. The el-cheapo bearings will warp the seal from the heat, spit the grease, and die quickly. I've gotten to where I rebuild my own idler pulleys because I don't like the el-cheapo bearings that some suppliers use. Warranty or not a failure every 6 months is unacceptable.
The real value in DIY is that if rebuild parts are cheap enough you can do it more often than necessary and never see a failure. Many parts like alternators and brakes have a predictable life span and trying to eek out every bit of the last 20% of the life will cost you more than the first 80%. Work on the car at your convenience and for $20 throw the parts away a bit early and save yourself the grumpy driver pick up, the cost of a tow, and the supersize service bill because you were away from home and couldn't fix it yourself.