Your plan is similar to mine as far as wanting one top notch lower to interchange multiple uppers with, so I thought I'd share my build plan. Whether you want to copy any of it or not is entirely up to you.

Lower Receiver: Seekins Precision SP15
I'd say you're good to go with the Mega. I don't have any experience with them, but I'm told they make an excellent product. In my mind, a lower is a lower, as long as it's made to spec, forged 7075, and Type III Class 2 anodized. I looked long and hard at the Mega, but bought the Seekins for two reasons. 1) It was in stock. 2) I like the way it looks.

Lower Parts Kit: Either a PSA or WOA + Geissele SD-E
I'm not yet decided on which parts kit I'm going with, but I know I want one sans trigger assembly. I would buy from Grant but his have been out of stock since I started looking, and don't appear to be coming back in any sort of timely fashion. That pretty much leaves me with PSA or WOA... I'll probably end up flipping a coin. I'm also considering sending off the major components to have them nitrided (not the springs/detents/pins, just the selector, catches, takedown pins, etc...), and my wife is left handed so I may end up swapping out the selector with an ambi version, although I haven't looked into it much yet. The trigger will be an SD-E, and that is only because I want the flat trigger bow and one of the uppers will be a semi-precision setup. I trust you've done your own research and decided the SSA is best for you. To round out the LPK I'll be swapping the trigger guard with a Magpul Enhanced and the grip with a Magpul K2 or a BCM Mod 1 if I get impatient.

Buffer System: Vltor A5 w/ Magpul ACS Stock
My research has convinced me that the A5 system is far superior to the carbine buffer system, so I've decided to absorb the extra cost for the added benefit. I'll be starting with the A5H4 buffer (heaviest they offer, with 4 tungsten weights) and swapping in steel weights from a standard carbine buffer if I need to lighten it up. Due to the added length of the receiver extension, I'm going with the longer ACS stock. You can never have too much storage space IMO... Even if you don't see a need for it now you'll have it later if you ever do. A Noveske QD end plate finishes things off.

Anyway, that's what I came up with for my "one size fits all" lower, YMMV. I would recommend looking into the A5 system though if you haven't already, and definitely this:
Quote Originally Posted by dentron View Post
dont forget the custom engraving that says "kick-ass lower"
On the right hand side of the magwell.