I'd also heard that KitKat was meant to be a lighter OS to allow it to run on older and less powerful devices - have I misheard that?
This is correct, kitkat is designed to work on devices with 512mb ram. However this does not mean that existing devices with these kinds of specs will get updated.
It is always safe to assume that any device under the $300 mark will stay on whatever firmware version it is released with. It is just not a high enough priority for manufacturers to update low end devices.
If you absolutely have to have the latest updates and not spend a fortune, your best bet would be to get a device with as much developer support as possible (Eg Samsung or Sony over Huawei or Vodafone branded etc) as they are more likely to have custom firmware developed.