I think some people are making this overly complicated for you. Buying an M&P 15 is an excellent place to start if the mods your husband wants to make are what you listed. I did pretty much the very same things to my M&P 15 Sport and except for the red dot I have less than $100 in parts. I have the M-Lok system on the front hand guard and I added a rail and a hand grip to that. Simple. And pretty cheap really.

I think you're right to stick with the S&W brand. Just jumping in to build your own weapon is going to require extensive input by your husband before any parts are bought. And then there's the warranty you get with Smith And Wesson stuff. I waited years to find the right rifle for me and this M&P 15 Sport was it. And I waited just so I could do the things you mention and do it with a name brand with a good reputation for the particular rifle.

So I would suggest finding the best price around for an M&P 15 Sport and then let your husband decide what parts to add. He won't be tossing that much IMO. Not for the mods you list. You should be able to find one of these rifles well under $600 these days. Here's an example for $500:

Smith & Wesson M&P15 Sport 2 AR-15 556 nato 16″ 30rd Black – GUNPRIME