You can use a lower from wherever. Aero Precision makes more than 50% of other manufacturers lowers today... they are a good option and can usually be had less than $100.
I would suggest buying a prebuilt upper and building out your lower as you see fit. It's easy and requires very little tools. The upper requires specialized tools so you won't save any money there - at least on yoru first build. You won't beat a Daniel Defense cold hammer forged barrel... so if you don't want to get a standard DD upper... you can get one of the uppers from AIM Surplus that include DD barrels. Definitely a good option.
Glock 19 G4 | Glock 21 G4 | S&W M&P 9 Pro | S&W M&P 22 | Sig P220 Elite Dark | Sig P226 MK25 | HK P30L | HK45 | HK USP 9 | HK USP Compact 9 | HK USP 45 TACTICAL
Daniel Defense DDM4 V4 | Arsenal SGL 21 | ASA SBR w/ Side Charger | Savage Mark II FV-SR
Mossberg 590A1 20" SP | Mossberg 500 Cruiser