Joe
I have just seen this post and maybe these 2 comments come a bit late;
1) No one has mentioned the aspect of your slope. You say it faces E-SE which presumably means you loose the sun in the late morning? If this is correct then I think this is a major problem as most veg require sun for most of the day (parsley seems to be an exception). Unfortunately, there is nothing you can do about this apart from possibly finding other neighbouring plots with a better aspect (and a less steep slope?). However, I think it worth pointing out as it adds to some of the negatives already expressed above.
2)I have grown my veg in raised beds for about 30 years on a (south facing) slope, but only of 10 degrees or so. The beds are across the slope and work well, and like terracing, only the bottom sides of the beds need building (and possibly the ends). You have material to hand (rocks) but as Charles says the labour involved will be v. costly on your steep slope. Maybe you can enlist some of your potential customers to help build a bed (there will need to be some expertise input in dry stone walling) in exchange for future veg coming out of “their” bed?
Sorry I can’t be more positive but I think it needs to be very carefully thought out and costed before you get too far into this interesting project
Dan
