Don't build an 'anti armor' kit.
Those are expensive to do right and your ROI will be very, low.
If you want to do it right, PM Wicked-Mpact and buy one of his AT4s. They're only about $400.
I have an Escort HK416, an Inokatsu AKM, a Mk14 of too many manufacturers to be pinned to just one, and a mid-length Classic Army M4.
I have never been at a deficit such as, 'Man, shit, wish I had ____________(this gun)_________'.
I had a Classic Army Mk46 sitting around for awhile which I never, ever used.
If I were you, and you wanted something different than what you had now, I would get an M14. They are going to be the most accurate, longest ranged platforms to do stuff with. I really like mine. The only problem is that the magazines are cumbersome and require special gear (you can use M4 pouches for AK, MP5, M4, P90).
However, airsoft is about having fun. On the same tier of quality, guns are all going to perform about the same, they're just gonna look different. If you want an LMG of some sort -- do it. If you want a small gun, do it. I don't like small guns because the range of your AEG is dependent largely on your inner barrel, and, small guns have small inner barrels. Unless you have a P90. Then, you have the unfortunate consequence of owning a P90.
My picks for a SAW would be an Inokatsu or RMW RPK.
My pick for a small gun would be a KAC PDW whenever those come out.