For your budget you have 3 choices -
1. Buy a sigma lens, either the 150-500mm or 50-150mm
2. Rent a lens, lots of places but the popular one here is lensesforhire.co.uk
3. Buy a used canon 100-400mm and sell it after your trip for very little loss, providing you do not intend to use it in the future.
Eithr way, I'd reccomend a trip to the zoo before you go with your lens to get acclimatised to it. Personally, I'd be tempted to hire one soon as they do take a bit of getting used to, but after a day at the zoo you should know if you're happy to lug one around on holiday