Let me answer some of your questions.
1. Your not going to find a transferable glock sear, most likely there all pre or dealer samples, SOT can only have them.
2. MP5, there are 2 ways to have a FA, RR registered receiver or sear, RR is the cheapest and less desirable, the sear "Fleming or ?" is the most versatile usually in 9mm, 223 and 308. The down fall of a RR you can't remove the sear and place it into another host, so if your RR is under 16" no stamp is required for a SBR.
The sear you can move it to any host you want, only down side if its under 16" every host must have a SBR stamp.
3. My suggestion look on subguns, sturm and GB, I will personally vouch for Ruben in FL, my rich buddy bough 2 MP5 from him and I bought a Ares Shrike Upper.
The process is exactly the same as a can.
After seeing the amount of $$ my buddy has spent on his HK collection, compare to my M16 its amazing all I buy is uppers while he buys complete guns..
The 16 is the best bang for your buck...