Newbie greenhouse questions

I’m starting a tiny market garden next year and want to focus a large portion of my offerings on veggie starts and some fruiting perennials. Not only is my actual ground space limited, being in a neighborhood setting with only an 1/8 of an acre backyard and large dogs that still need their sprint space, but I really want to help encourage other people to grow their own food at home and find some independence in their food supply.

I’ve been trying to focus more time on researching greenhouse do’s/don’ts and trying to come up with a solid plan. From what I’ve seen so far a ready made greenhouse would be out of the budget since I’d need at least a 10’x20’ so I’ll be needing to build something from scratch. There’s soooo many options and everyone seems to swear that their way is the best way, I’m just getting a little bewildered. I really hate diving into a project just to realize things should have been done differently, trying to avoid costly mistakes upfront since this is such a large part of my plan.

Of course my dreams swirl with the fantasy of a victorian orangery bordered by cobblestones and French lavender, but reality screams plastic sheeting quivering noisily in the breeze... So far I’m leaning towards bending cattle panels and attaching to a base frame, but I’d very much like to incorporate automatic ventilation to avoid roasting things when we get sporadic heat jumps, don’t know if that’s feasible with this design though. Is there a consensus of tried and true methods that many of you would agree on? Trying to hammer out a solid plan with no experience is proving a bit maddening. Calling all brains to be picked 🙏🏼

I find tomato starts are the key offering, and those need warm conditions to get started, so you'll likely need to do them inside anyways (not sure where you are though....).  A propagation cupboard with lights might be a better place to start (?).  Don't stress too much about making mistakes, that is the best way to learn!  For greenhouse, just get started with a budget DIY design - cattle panels, plastic pipe or rebar frame with plastic sheeting and think of it as a learning opportunity rather than the end solution. Base the design on the resources you have available to you most easily - ie maybe the cheapest option is actually building with scrap wood and old storm windows you already have lying around, rather than buying a bunch of new plastic pipe/rebar etc.  I have a small A frame greenhouse built with wood and plastic vapour barrier that my ex made.  It's not big enough and I don't like the design that much (not enough headroom when the tomatoes get big) etc etc but I've had use of a greenhouse for several years now, so I'm grateful for that and hopefully sometime soon I will get it rebuilt in a better way.  

Welcome Sadi.
From what I gather greenhouses seem to vary a lot depending on what your climate is like. I follow a few youtubers with greenhouses, one (in Arkansas? not entirely sure) I can recommend you check out with several greenhouses (including a high tunnel and a thing made of demolition windows) is Roots and Refuge Farm-- and the family use it to grow tomato starts to sell at the farmer's market, might be interesting to you. https://www.youtube.com/results?search_query=roots+and+refuge+farm+greenhouse

Ah yes, my area might be of some help! I’m in southern NC, bordering right on zone 7b and 8a. I could do a small solar panel that would run an efficient space heater through the nights. I’ve also heard that a few moist straw bales inside help to put off ambient heat as they begin to compost. I’d like to get an earlier jump start so my plants are a bit more vigorous and I can grow out some larger ones at a higher cost for those that only want a few and more immediate results. Our farmer’s markets are pretty saturated so any leg up having larger starts to offer would be a plus. Definitely going to be doing a wide variety of tomatoes, plus cucumbers, peppers, mellons, and squash, then swap over to cool season crops late in the summer to help convince people that a fall garden can be just as rewarding as summer.

Unfortunately indoor space in the house is at a premium, I have six cats and they will find any opportunity to squeeze in somewhere to grab anything green. I lost two flats last season despite keeping them in what I thought was a safeguarded location, just to find my fat boy got over the baby gate and climbed up to snack then kicked them off for a nap on the heat pad 😑