As Busy-Lizzie has already said. The main concern is protecting the property from damp while not affecting the plants. There are many many books, blogs, websites et al on vertical gardening available (believe me - I've read quite a few of them and they vary from growing carrots on a balcony for fun with your kids to growing your entire family produce in the basement to sustain you when the zombies come).
Other things that would be limits (for me) are:
- What space does the plant itself need to grow? Your design wont be any good if it is too small for produce itself. A potato needs completely different things to lettuce. Perhaps the idea of a "farm" is a little too grand a title and your aims need to be limited to certain products (i.e. tomatoes / peppers / dwarf trees / produce that is above ground rather than carrots and potatoes). Research what kind of produce you expect it to be used for so you know what amount of soil and growing area you have design for.
- How will you keep the plants watered while at the same time protected from water logging, drought, and frost? If you're taking advantage of the height of a wall will your end user be able to water the plants with a watering can or will they need a inbuilt system that waters the higher levels automatically? Will that plumb into a watering butt that forms part of your design or will they then need to provide an outdoors tap? Will they be able to water some plants more than others easily as not all plants want the same amount of water?
- Looks. Its got to look nice even when the plants are still seeds and not able to cover it over. A product is a complete fail if not matter how brilliant it is no one will buy it.
- Maintenance & use. Aside from giving the plants water how easy is it to actually use? In a bid to go small and virtical don't also make it very difficult to add compost too (for example the pots are too deep the average person can't reach all the way in) or too high that you can't then pick your produce.
- You've used the word farm. Are you intending it to be a product that is used for one harvest or will you be able to grow different plants at the same time that can be harvested throughout the year? Make sure it is easy to harvest one crop without disturbing another.