Not long ago I was talking to a relative, an older gentleman who bought a M&P Sport AR a few years ago. It is his first and only AR, bought for security/defense more than an interest in ARs. Apparently, he was reading about ARs in recent times because he noticed and took a liking to Daniel Defense. That is until he saw how much they go for. I was telling him he can change the trigger if he doesn't like it, or barrel if his stock one does not give him the accuracy he wants. I may have mentioned another thing or two for modification I don't recall. He said mods would hurt the resale value.

Since the conversation above, I thought: He can buy an upper if he likes one and put it on his stock lower. He can have an armorer do it if he's uncomfortable doing it himself.

I feel safe to say he would not want any machining done on any part of his lower or upper. Are there any upper brands that go especially well on a S&W lower? Are there any upper brands that he best avoid?

As for the trigger, can an armorer or 'smith smooth out the trigger feel and lower the pull weight using the stock parts he already has in his lower without much risk of butchering it?