That's a very cool design! Be sure to post pics when you're done!
For the heater, depending on the average air temp around the tank, you could get away with a 300 watt (I don't think I've seen a 400 w). If the air temp around the tank is significantly cooler than the water temp, you might want to consider two heaters, one on each side. I have a 300 watt in each of my tanks, and they don't get too hot. I've even accidentally moved one around before I realized it was on. The heat disperses very quickly through the water. However, if you're concerned about it, I've heard of making a heater guard with a piece of PVC pipe with lots of holes drilled into it.
I don't know anything about pond filters, but I have a fluval FX5 in my 90 gallon that I love! A 100 gallon stock tank would be a good candidate for the FX5. Just remember that with canister filters, the water level needs to be above the filter, the higher the better. A foot should off the ground should be OK. I have my tank at about 18" (2 cinder blocks high).