Personally I wouldn't run a sump with a planted tank as if were wanting to run CO2 at any stage it would make it less effective.
I'm not familar with the Eheim 2229, so cannot comment on its heating ability. However, what I can tell you is from my experiance, external heaters (the ones you plug into the in/out pipes on canister filters are far more effective than just simply just having a heater in the tank as they heat the water as its being circulated around the tank.
Ive had my Hydor 300w external heater on a 500L tank running at 30c without any issues.
The heated canister filter therefore, might be worth a crack...
Your other options could be to get a normal canister, and buy a hydor heater, would most probably workout cheaper, and if one of the items failed you wouldn't have the issue of having the issue of tearing down the filter to get to the heater.