I am looking to build a Mk18 SBR (well aware of NFA procedures) and would like to ask for some suggestions. What stripped lower would be best to use aesthetically speaking?  

Any other suggestions?

Thank you in advance.

CSP

The most accurate COTS lower would be a Colt M4A1 Carbine lower.

Go look at as many pictures of MK18 clones as you can.
Go read all the MK18 clones discussions that exist on the internet.

Then, buy the lower that you think is best.


It's that easy.



_

Just read the entire internet! Then come back and realize that a Colt SOCOM was the right answer :\.

It also sorta depends on what you mean by Mk18. If it's a CQBR Block I or Block II, then as stated, the Colt 6920 SOCOM lower with "M4A1 Carbine" markings will be the closest thing you can get off the shelf. Those are fairly pricey and hard to come by, but it would be the most "realistic".

If you're talking about the original Crane Mk18 Mod 0, then it'd probably be a Nodak M16A1 marked lower. Maybe. The M16A1 lower used to be considered the lower for an Mk18 Mod 0 (and maybe it still is), but I guess they might have been less common than people think.

Probably the best answer for either, and the closest to "clone correct", would be that a Colt SOCOM. Or go all out and have a blank lower custom engraved.

Personally, I wasn't willing to spend $400-$600+ each on Colt SOCOM lowers (or whatever they are going for now), so I'd say that any mil spec lower will work. A regular 6920 with "M4 Carbine" markings would probably be a good second choice. I took the stance that once you've decided not to go full "clone correct", just get whatever lower you prefer.