I would probably look at EGW ones. Think they make them that come with spring and keeper. A good place to start with is whatever spring rate the gun came with and go from there.
Full length guide rods are mostly used in guns with bull barrels.
In my guns I have never replaced one for another...
I could probably build 2 complete guns with my extra AR parts. Mostly parts and stuff I've upgraded and replaced over the last 40 years. Yes, I bought my first two in the early 80's, both were FA guns for under 2k each.
Essential parts for me are extra lower spring kits and probably 2-3...
Yes, it's tough. Had to deal with my Mom, Grandmother and Dad all passing within 9 months. Cost me my job having to take off a lot to deal with things.
9mm for us, both in PCP and handguns
All use Glock mags. 2 Glock 19's and a AR9 and a Draco NAK9
Numerous other guns in 9mm
Do have rifles in .308, 5.56 and 300 blackout