I'm not sure that I completely agree with fundraising to walk up a mountain (holiday!), but fair play to you for getting out and doing something. At least it raises the profile of the charity.
Kilimanjaro isn't really that challenging though - depending on which route you're taking. If you're doing the Coca-cola (Marangu) route then its really just four days of gentle trekking. I did this route as I couldn't afford to do a more challenging one - they take longer so the Park fees are more. If you can do a weekend of hiking in the Lakes/Snowdonia/Scotland then you'll be fine - the problem is really altitude, and the company you're with should make sure that you don't go too fast. Your base fitness level should help with that too.
If you get a chance while you're in Tanzania - do Mount Meru (second highest in Tz). Great walk through the National Park at the base, with all the animals around (and your guide with a gun) and a fantastic view of Kilimanjaro from the top. Makes for a good acclimatisation walk as well.
