I have a dream... to build a couple AR15 lowers in a few different configurations and have a fairly large collection of uppers that can go on almost any lower. The point is to be as low effort as possible to convert to any caliber so each upper will be prebuilt in its caliber with it's own bolt and will just be a drop-in/plug-and-play set up. Currently, all I have is a M4 type lower (Carbine buffer system) registered as a long gun. I plan on building/buying, 1: A basic pistol build for rifle calibers, 2: An carbine set up with a high grade trigger, 3: a basic pistol build but will only have pistol caliber conversions in the mag well, and 4: a dedicated .45/10mm glock pistol lower. I currently have no concerns for lower builds 1 & 2. The caliber conversion for rifle calibers is very straight forward (mag/bolt/barrel and/or mag) and with currently 4 calibers it has been a success (5.56, 5.45x39, 7.62x39, 6.5 Grendel). I am looking to add at least 3 more calibers to that collection (6.8SPCII, .458 SOCOM, and a magnum rimfire) and then have some pistol length versions on some calibers. My primary concern right now is lower builds 3 & 4. I want to continue this plug and play mentality to the pistol caliber guns. For example: have a .45 conversion in #3 lower and have an upper that will work with both it and #4 (dedicated large glock lower). I have tried to research and figure out compatibilities and incompatibilities but so far I have been stymied. Does anyone have experience with these products and can give advice on this? Is there a resource that I missed? Advice?

Google and Bing are the greatest resource available to you to search for you want.

Advice......spend your  time reading gun forums, specifically, the threads started by others that have done what you want to do.

MGI modular lower....

Yep. MGI allows you to configure one AR in a variety of calibers.

I have mine set up to shoot 5.56/.223, 300 BLK, .22LR, 7.62x39 (with AK magazines) and 9mm (using Glock mags) and working on 5.7x28.

Great rifle.