We're renovating an old house and the HVAC is at the end of its life cycle, so we've gotten a couple of quotes for new everything. (2) - 2.5 ton air handlers (up and down), (2) 70k Btu gas furnaces, all new ductwork up and down, etc.

I have a few questions regarding the quotes I've gotten, as I don't know much about the info on here and I need to know when it's better to upgrade vs take the lower-end quote. i.e. Will the differences be noticeable or negligible?

1. SEER rating - 13 SEER vs 14 SEER & up - is the difference negligible as far as cost savings, comfort, etc?
2. variable speed vs. multiple speed - what does this mean to me?
3. heating stages & cooling stages - 2 vs 1 - what does this mean to me the user?
4. dehumidification - yes or no - doesn't having AC automatically act as a dehumidifier? do i need it as an add-on?

Also, what is the difference/advantage/disadvantage of using galvanized sheet metal ducts vs flex ducts?

Thanks for any help or insights!