I want to powdercoat a block I have. It has some rust and is grimey. After it is hot tanked and checked for cracks, should I have the block sandblasted (might be too harsh) or "media blasted"? Then, how much would powdercoating the block cost?

You will never really see the block, so I would just hot tank it rough it up and spray it whatever color you want... I did mine black and its still holding up fine.

Of all the blocks I've seen get built lately they've all been painted with a spray can and look and hold up just fine.

Even just cleaning them up and painting them with a base/primer coat followed by a good high temp engine paint works wonders.

What about sandblasting/media blasted to get rid of the rust? Will the loose shit just come off in a hot tank?

You could sandblast it as long as you were very careful to stay off the machined surfaces. Personally, I wouldn't do it that way, there are chemical rust removers that will work fine and be less dangerous to the block. The other thing to consider is that the paint work should be done after machining and as soon as the block is hottanked, it will rust again pretty quickly. I always have a bit of clean up to do when they get back from the machinist.

What about just a wire wheel or a roll-loc to take the rust off.