You'll have to find the balance between displaying the Ad content and interfering with the user experience.
Would you be happy to go out touting for one or more Sponsors yourselves? or would you be better simply using one of the existing Ad Networks like Google Adsense out there?
Personally I'd recommend the latter as it takes much of the effort away from you. If parts of your site are hidden behind a login, it's possible to provide Google with some credentials so it can scan your site and assess it in order to display relevant ads.
However all of this is dependant on you driving a significant volume of traffic through your site. 20 visitors isn't going to generate much of a return if only one in 200 visitors actually clicks on an advert. The more relevant the ad, the more likely a user is to click it, and the more users you have seeing an "impression" of the Ad, the more clicks you could get and therefore more revenue.
Just be aware that Google has some limitations on how many adverts can be displayed on a page which (in my estimation) make it reasonable without being too intrusive to the purpose of the site.
Just my Ha'pennies worth!