Cracked casing on back of laptop

I agree, it's a difficult procedure, requiring transplanting pretty much all of the MacBook's components to another bottom casing. Although, it is do-able, and iFixit has a guide, so you can see what you're getting into. The bottom casing can be purchased on eBay and probably iFixit for a fairly reasonable price, however there are a couple different versions, so it's important to make sure you get the Core Duo bottom casing and not the Core 2 Duo casing.

If you decide to live with the casing as-is, perhaps some heavy duty clear plastic tape over the cracks would alleviate the concern about dust and moisture.