Depending on whether or not you want to go thru the process of ordering a lower there's a few different ways to do it. You also need to find out if there's even stripped lowers available in your part of town.
For fun, we'll assume you know of a gun store that will help you receive a stripped lower.
Look somewhere like here:
[url]http://www.ar15.com/forums/topic.html?b=7&f=21&t=172211[/url]
or here
[url]http://www.ar15.com/forums/topic.html?b=7&f=21&t=165943[/url]
you get the idea.
Coordinate with your friendly local ffl to receive it.
Now to go parts kit shopping. You can go chrome lined bla, bla, bla, but the sad fact is that your husband is not fighting a war of attrition in a jungle, so don't worry about it.
There's numerous big parts houses:
[url]www.jtdistributing.com[/url]
[url]www.m-aparts.com[/url]
[url]www.del-ton.com[/url]
Click thru there till you get 20" parts kit. If you can find it, get him the "Gov't Profile" 20" bbl. It's lighter under the handguard and makes for a better all around rifle.
In between all this you'll need to decide if you want an A1, A2, or A4 upper.
I think this is the short answer so if you need any help, ask here, via IM, or email [email]
[email protected][/email]