funny you should ask...
i have been doing alot of research lately on that very same subject. we determined here, that we would sell a gyro on our menu, but limit our greek food to that.
but, the key is location. and of course, you should do the gyro from scratch. its not that hard.
http://greekfood.about.com/od/greekfoodphotogalleries/ig/How-Greek-Gyro-is-Made/Gyro-Rotisserie-Grill.htm its likely less cost effective this way, but absolutely the best product.
the marinade most likely includes some sort of curing agent that keeps the microorganisms at bay, while the cone is in the temp danger zone. that's just my guess.
we found a warmer for a prefab cone for only a thousand dollars.
what you would want, is one that sits on the counter, or in a bay wndow, to draw people in from the sidewalk. the sauce, is pretty easy to make as well. of course there are variations on the prep method, but you can decide which way you are going. ours is something like this...
1 pint sour cream(or Greek-style super thick yoghurt)
1 English cucumber, peeled, grated on a box grater, salted lightly for 5 minutes and squeezed between the hands to remove water(the long, thin, almost seedless kind)
3 garlic cloves, mashed to a paste
1/2 cup olive oil
1 tablespoon red wine vinegar
1 tablespoon minced fresh dill (optional) or fresh mint. or both
salt
Directions
1
Just mix everything up together until it is all blended and the oil has emulsified into the yoghurt/sour cream. Taste for seasoning, and add salt if you think necessary. Put in a resealable container.
2
Allow to 'sit' in the refrigerator for at least 2-3 hours before using to allow the flavors to come out( or over night) has a 7 day shelflife
3
This wonderful meze. it is a thick 'salata'. it is used to counterpoint rich, bbq'd meats (like souvlaki/shish kebab), but can be used in more diverse ways.
the pita bread can be obtained from most supplyers. easy squezzy.
now the big problem that i have with you business plan, is the labor pool. you will find that to be the biggest obstacle. you may have to jump in there, and go part time with your day job. no one will make this great food or serve it like you would, as the owner. plus, without you there, many gyros would go out the door unpaid for(for friends), not to mention tons of other possible complications.
so, i think it would work but i think you will have to commit to being there.
peace out
mike