greenhouse Green House / High Tunnel Platic

Have build many small green houses / hot boxes before, but nothing large enough to walk into and always with the sorta clear / sorta white plastic from the hardware store.  Have never purchased plastic that is actually made for greenhouses.  Looking for advice.

Goal is to extend the season in the fall, get an early start, and have another place for isolation in the summer / fall.  For isolation, will be leaving the plastic on the hoops but opening up both ends and covering with bridal cloth.

What would be a good general purpose green house plastic?
I'm also interested in this answer, I need to build me some type of tent / walk-in area to contain my tote + some buckets maybe.
Juanitos, thanks for the link.  What confuses me are all the different films.  By giving a link to the standard 6 mil clear, I think that means you are saying go with the basic stuff and screw all those fancy things?
BTW: Price at the link seems real good to what I have googled.