I'd say no to the extractor mod.
As far as spare parts, a spare bolt should pretty much cover you. I'd buy another DD if I were you or at least make sure whatever you get is MPI.
RE Lube...there are a million threads here about it and some guys change lubes more than they change their underwear. I started using SLIP2000 last year after using CLP for the past 10 years and the SLIP is much better. It doesn't deteriorate nearly as quickly as the CLP used to.
If I were you, I'd get 12 PMAGs, a good Carbine Course, and 1k rounds of quality brass cased ammo before I worried about buying a spare bolt, though. I'm telling you, a basic carbine class from someone like Vickers will answer many of the questions you may have as well as teaching you good weapons handling, manipulation, and marksmanship techniques.
Semper Paratus Certified AR15 Armorer