Shouldn't I wait on everything else until I have the lower striped and kit?
If you are dead set on building, I would not pass on any deal. My uppers I ordered are both en route, I have LPKs already, but other than ordering them, the lowers are nowhere to be found. Could be weeks, months before they show up for all I know. Thats why I am still looking for ones in stock.
If I want to have the option of Mil vs. Comm spec do I need to avoid certain lowers?
Maybe some, but not really. The stripped lowers that you will see moving forward will all be fully capable for any build or model you want. If not, people will be quick to point it out when they link it.
What about the M4 option? Avoid certain lowers as well?
Same. You shouldnt have to avoid any metal lowers you see in here or that link, they will be more than capable of building whatever you want.