Are you collecting, building a set of self-defense tools, or adding competition weapons?
1) Carbine
The M4, MAK90, and SKS are all great for close range rifle fighting.
2) Pistols
The XD's and Kahr are great for carry.
From what I can see, you really have everything you *need* to defend yourself. Have you considered getting formal training in the use of these weapons?
The only "holes" I can see are for long range shooting. You may want to look at a battlerifle such as a Garand, FAL, or M1A. These will get you longer reach and higher capacity. You should be able to score an M1A for around 900. You should be able to get a good Garand, ammo, and bunch of clips for $900.
You could also look at getting a bolt action rifle with scope.