Congrats on the property and good luck!
I have some experience with work exchange and these are some of the things I ask/think about when determining a fair trade. To determine a "fair" exchange some more information on what you are offering and what you expect in return would help. Things like living accommodations, expected hours per week/month, length of stay requirement, will you supply any foods like staple foods (rice, beans, etc) general farm plan (what skills/experience will I be able to gain)
Because an educational opportunity seems to be the main thing you are offering in exchange for the interns labor what is you're experience and knowledge. You said you have a 1/4 acre urban permaculture site that you've been practicing on for 5 years. Have your studies been in higher education settings, workshops, self taught, PDC, etc? How involved will you be in educating your interns? Will you be working alongside them or assigning tasks? Personally I'm wary of educational exchanges because I've come across folks who are either A. Not as knowledgeable as advertised B. Lack the skills/focus to teach C. Just want someone who will do the hard labor/property maintenance tasks. I'm not implying this is your intention, but these are things I've come across.
Also knowing what kind of permaculture site you are trying to develop would be helpful to attract the right intern. An educational site, a homestead, market garden, CSA garden, U-pick gardens, will you have animals, etc. Knowing these things will help to give the intern an idea of what they will be able to learn about.
If you aren't familiar check out WWOOF.org and Workaway.info these are work exchange sites that might give you some examples. Attra.ncat.org has a page listing internships/apprenticeships all across North America. In my experience WWOOF and workaway are generally people who want to travel on the cheap and also maybe gain some experience along the way while Attra generally has people who are seriously interested in farming and want to learn.
As for a different approach if you were interested in attracting people with experience for a longer term commitment say 6 months to a year+ to help you develop your property depending on your farm plan and potential income streams from your property setting up some kind of profit sharing or work exchange plus a stipend could help you set that up.
This post became way longer than I had anticipated. Good luck with everything and I hope this helps you out.