- What countries do you need traffic from?
- Do you require "a spot" on a site, or could donors just give X impressions where X is whatever number can fit in around commercial banners?
- I don't think I've ever come across 250x100 adverts before, why this size? And more importantly, why only this size, would you not be interested if people could offer you MPU, Leaderboard or Tower impressions?
edit:
Regarding how you serve adverts, I wouldn't recommend creating your own platform. We've got an in-house system that works well for us, but if we hadn't put such a huge amount of time/money into it, it really wouldn't.
Google offer a free solution for small business with less than 90m pageviews (http://www.google.com/dfp/info/sb/index.html) and might well offer it for higher numbers for non-profits, I'm not sure. Alternatively there are open source off-the-shelf solutions such as OpenX (http://www.openx.com/publisher/open-source-ad-server).
edit 2:
You should also publish who your clients are, to give an idea of what charities you will actually be supporting.