Hey guys, been looking around, great place you got here.  A couple of newb ?s

I really want to buy/build an M4.  I want it to be preban due to configurations.  What kinda price am I looking at on this deal to build?  What is reasonable price to buy?  I have found several prebans on gunsamerica ranging from $1800-$2500.  Also, this may be a dead horse kinda question, but I'll ask anyway.  I know you cannot put preban uppers on post ban lowers, does it work the other way around, can I put post ban upper stuff on preban lower?

here are my plans
M4 with flat top
Aimpoint CompML2
PVS14, in about 6 months
some of the GG&G stuff for mounting
Surefire light with bite switch

Thanks in advance fellas, again, great site

What configurations are you interested in?

The Lower receiver of the AR15 is THE gun according to the BATF. So if you have a lower from a pre-ban gun, then it is legal to put any config of legal length (barrel at least 16")  you want, pre or post ban.

hope this helps