FYI: Picked up a BM 21S at the local gun show and WOW is it nice.  Thought about doing a build but $'s weren't there to be saved against the BM.  For those on the fence, these were the numbers I had to work with.

A3 M1S kit is $470 Non CL
A3 M1S kit is $505 CL

Add $18 shipping per M1S

Add $125 for a lower

Add $75 for a rear sight

Add $15 for a 30 round GI mag

and my total would have been

$703 Non CL OR $738 CL

The BM was NIB $790 OTD and came with SS bbl, rail, sights, handguard, pistol marked lower(I already know it doesn't matter), factory made, factory warranty, comp & teflon coated 30rd mag.

Building myself would have saved me $83 max from the best prices I could find and I would have been on my own.  Unless someone has specific needs building doesn't seem to pan out $'s wise.  BM customer service and warranty are well worth $83.