Given the kit lens of the 400D, you can either:
get a modern version of the 18-55. But it's a bit of a minefield, because Canon reintroduced a non-OS version for the low-end cameras. So make sure you have IS, IS II or IS STM (on a 400D, the STM isn't very useful as a "video" lens but you may enjoy the non-rotating front lens). Since these are kit lenses, many are available on the second-hand market if you want rock-bottom prices.
get something a bit more expensive but slightly faster and with more reach, like the Sigma 17-70 f/2.8-4 (either the IS, or the IS contemporary)
- or something faster with less reach: a 17-50 f/2.8 from various brands (Sigma, Tamron)