I have 35W's in all mine. If you get a colored 55W, the higher watts will wash out some of the color. I heard some people have issues w/ the 55w, but don't know anyone personally who has.
As for color, 5K will give you the brightest white. I have 6K in all mine and will give you a slight...
Check KBB and see what they list for Private Party and Dealers. Use that to negotiate price.
Between the two, I'd prefer the hatch. I think the hatch was refreshed in 07'.
Jesus Christ those ballasts are huge. I'd get a set from DDM tuning. $30 + shipping. Probably comes from China, but it's all plug and play. I've bought 4 sets for various cars and have had no issues.