PSA would be a good choice for the LPK as you said. If they have a Magpul stock/LPK, those are a great buy if you like the stock. I think they have a CTR version for a great price.
My M&P-15 lower has a Daniel Defense LPK (very good quality but twice the cost of PSA even if you can find it). And DD upper. I call that rifle a Dan Wesson.
S&W makes a great stripped lower.