Codex WH and DH have rules for allying units around. They're kinda complicated, but 'ard boyz has a good summary of what you're allowed to do in their packet.
As for the Land Raiders, since 5th ed came out I've more or less exclusively played Grey knights, and I've always spent between 25-50% of my points on Land Raiders. It's rarely been a problem.
In fact, it's more of a problem to spend fewer points on Land Raiders, I've found. In games with only one (or at this points level two) land raiders, and relatively little other support, they die easily and accomplish little. Taking 3, however, is very effective in such a high points game.
Plus, GKs need Land Raiders. As in, they cannot play competitively without Land Raider spam. They simply don't work competitively if you don't take lots of Land Raiders. This is, after all, the codex that's been at the very bottom of the pile since long before Necrons got nerfed when 5th came out.
Very rarely, there is an occasional game where the opponent just gets ridiculously lucky and blows up a bunch of Land Raiders in a row. But at this points level, I have enough other stuff where that can happen and I still have a chance of winning.