My son was 13 when he built his for a set of parts we bought him. Lots of videos on the Internet to point you in the right direction. From what it sounds like you might want to build one as it will teach you how to adjust or tweak it. Not sure how many guns smiths around you know the AR platform.
It is a fun platform for more expensive lead slinging.
There are some differences between the base specs and models of each AR you listed. The m400 has a 1:7 barrel twist while the 15T has a 1:8. The sig also comes standard with an ambi mag release and upper to lower tensioner screw; the m&p doesn't.
You really can't get the same front sight and rail package on the m400 as compared to the 15T. Every model had an A2 front post except the SRP. If you went that route then you'd have to buy a quad rail separate since the SRP comes with a standard fore grip.
Decide what's important to you since they are both around the same price point. Google is your friend as there is a ton of info already out there. I have the m400 and m&p 15 and they are both great shooters with 100% reliability with about 500 rounds through each.
On a side note, I was at Walmart this morning and saw a m400 enhanced (Magpul furniture) for $996.
New for me !
7 months ago I buy my first rifle, the M&P 15-22!
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