I can't answer all the questions, but I'll help with what I can.
AR lowers are still out there for a good price. Anything under $100 is a decent deal, depending on how complete it is. You can buy "raw" lowers for about $40, but they require full machining. The most common lowers are called 80% lowers. They are almost fully completed, you just need to finish them off, usually it's drilling the trigger group holes. Anything over 80% is considered a functional part, and requires a serial and FFL transfer. You can buy 100% lowers with serials.
Keep in mind, as an individual, you can buy an 80% lower and build a non-serialed gun for your personal use. However, if you try to sell it, you are now considered a manufacturer, and would need a manufacturer's FFL license and add a serial to the gun.
Quality wise, I can't comment. I don't deal with many. It's just so much cheaper to buy a complete factory gun nowadays. Every brand sells a $550 complete model.
As for aluminum vs poly... I have never seen a poly lower only for sale, but if they exist I'd stay away from them. Windham Weaponry is the ONLY manufacture that has perfected poly/carbon ARs. All the others have issues.
Tool wise, an AR multi tool should do most of what you need, depending on what you are doing for an upper (assembled upper or scratch built).
The other thing to consider...many times mismatched uppers and lowers don't fit together well. Another reason I like a complete factory gun.