Nashbar.com has Shimano Deore front and rear mechanical disc brakes on sale for $40 each. I don't know how good they are because I've never tried them but they look decent.

Use dry chain lube for mountain bikes. dirt doesnt stick to them as much as wet lube. the downside to dry lube is it doesnt last very long. relubing after each session is not uncommon. but you also don't need to clean it as much as wet lube. because when the dry lube flakes off the chain, the dirt goes with it.