RO/DI and other random questions

[Sea Witch]

I have a few questions so I thought I'd lump them all together.

For buying an RO/DI unit,

1. What should I be looking for?
2. Is there a big difference between brands?
3. How much should I be spending for a 100gpd unit?

and unrelated,

4. What kind of filter do you folks use on an H or Q tank?

Thanks