With regards to the 300D, even if a stock lens does not fit, there are adaptors available that are not expensive... Jessops does a range of these and they also do have second hand lenses in most of there stores if you want to shop for bargains. If you are not worried about make etc.. then look at Sigma lenses.. even new they are not very expensive and it is not like you need to rush off and buy a new lense every 5 minutes.
Hunt around on the web... there is a firmware upgrade for the 300D which will bring it very much in line with the 10D - This firmware updrade has had very good write ups in all top photography magazines as well.
I'm having trouble finding any second hand lenses. I've tried all the shops near me, and they have nothing vaguely suitable. Can anyone reccomend an online 2nd hand photo store? I think I may just go for a Sigma 17-30mm, but that's still alot of money. If only I could afford the Canon 17-40L
I don't see any need for the wasia hack just yet. I don't think the MLU would make any difference unless I'm trying to shoot the moon at 500mm (I'd be interested if anyone thinks otherwise) , and I'm not tempted by ISO3200