Help me with my sump

[Chief Hill]

I am picking up a sump made for a 29G biocube this week. So to help others in the future I will take picures and do a build on the process for others that have the same issue as myself.

My issue is since my tank is set up and running I don't want to drill out the back. So my option is a HOB overflow with U tube.
Eshopps Nano overflow offers a decent box but is only rated at 200 gph.
Is that sufficient? or is there another type I should look at instead? Also I don't want a HOB that will have my tank 6" away from the wall. As it stands now the tank sits flat against the wall.

Also my other concerns.
1 Ensuring my return rate is the same as the flow rate
2 Power distruption (although very rare if it happens how do I ensure the sump does not overflow?)
3 a HOB that takes up little space to keep the "nano appearence".