Well said about the mods. Brakes are probably not worth it with the 4k, might be useful with the 5k. Probably best just to get some new rotors and pads, new brake fluid and a good bleed. Didn't see the part about the diff there, wouldn't bother with it.
I think the most useful things you could do would be to lower it, swaybars, bushes, get it sitting on some bigger, fatter, sexy superlites, new shocks if it needs it and any new steering components if it needs it.