A brief reply to your list…
The site sounds ideal. Tunnel needs full sun, any orientation is good, in winter an E-W tunnel picks up more of the midday sun; some writers say that this orientation is too hot in summer but I find it is fine in Somerset… and practically speaking the best orientation is up and down the slope rather than across, also that one end is close to your point of entry, and indeed some water (three quarter inch hose is better than half inch).
Straight sides are worthwhile for your reasonably sheltered site, I have not bought a tunnel for years…. Beds can be created before or after erection, whichever you have time for first. I suggest starting with an 18×40′ tunnel and up to a quarter acre of outside beds. If as I suspect you are starting from grass and soil is loamy, a couple of inches of well rotted manure or Dimmer compost will be enough extra goodness but it depends how you plan to make beds… you could mulch now with black plastic (old silage sheets perhaps) to have clean soil by late autumn.
Good luck.