Right, you can put ANY 12" logic board you like in it as long as you get the matching DC to DC board for it. The part numbers are different for all the DC to DC boards even though some of them look identical. I think I managed to use an unmatched one once, but I don't recall which version it was from or which it went into, so just stick to getting a matching pair. The DC to DC board is not actually that expensive if you have to buy one. Its the board the battery connects to so don't go and buy another DC-in board as these are all the same.

Solid state drives are coming down gradually. I was quoted about $350 for a 32GB SATA 2.5" one just today. The 64GB version was double.
I imagine the ATA ones might actually be a little more expensive since they should be rarer.
Has anyone seen any benchmarks for the drives in the MacBook Airs? I really don't see how a parallel ATA bus can give you any speed increase using SSD over mechanical. Unless you are happy paying a massive premium solely for reliability, I would stick with mechanical drives. For the price of two SSDs, you could fit a pair of 500GB mechanicals and mirror them. (You'll need the new Samsung 9.5mm ones)