I use an external canister too - easy to maintain with loads of media. I only clean mine once every 3 or 4 months.
Undergravel are not good for plants generally as the action of the water flowing past the roots flushes essential nutrients. Generally the pea-gravel commonly used is unsuitable to as the grain size is too big. 1 - 3mm grain size is ideal for plants.
The filter shouldn't agitate the surface too much either as it drives off CO2.