I have this model for my 55 gallon tank

http://www.amazon.com/Aqueon-Heaters-Submersible-Aquarium-100-Watt/dp/B003C5TPF6/ref=sr_1_1?ie=UTF8&qid=1437133642&sr=8-1&keywords=aqueon+heater
I will be using an aquaclear(maybe two) to filter the tank.

Is horizontal, near the bottom, in the middle a good place?

As long as you have good flow around it, it doesn't matter.

Would one aquaclear filter(70 or 110..not sure which one to go with) provide good flow for an entire 55 gallon?

Never had any problem were ever I put the heater. I have had them in all kinds of positions. I have had moisture in them when laying them horizon but it didn't make any difference.

I just stuff the heater by the intake on the HOB filter and it draws most of the water right past it and does a good job keeping it fairly uniform.