The bulkhaed is what acts as a union between your display tank plumbing (durso standpipe or just a plain screen) and the pipes running to your sump it also has a gasket to prevent it from leaking so you don't have to silicon. Actually your better of not to silicon this gasket. The silicon can tear the gasket if you try to remove it afterwards. here's a link to help give you an idea:
http://www.bigalsonline.ca/BigAlsCA/ctl3664/cp18014/cl0/fittingsbulkheads?viewType=Category#
I'll try to get a photo of my tank to show you I just don't have one right now.
you can chose between inside or outside threading for the bulkhead as well, the choice is up to you. Just make sure you can get all the parts you need before you order. Surprisingly enough you local hardware store my not have everything you need. Also only use PVC piping.
The placement of your hole/overflow box, imo would be best to centre the overflow on the back panel that way you don't have to see the standpipe when viewing through the side.
As for the return pump that is again just preference. Take into account the amount of space you have around the aquarium. If the space is limited you might want to go with the internal pump and if that is the case make sure you leave enough room in the sump to house it. Given the size of your sump I don't think that should be a problem lol.
There are some draw backs to the internal pump such as they take up space that could be used as water volume and they can add heat to the system.
Have you thought about how your going to run you return lines into the tank? You could run some up to come through the overflow box which would hide some of your pipes. I think you would like to have more than 1 return line. 1200gph is a lot of flow to come out of one 3/4" opening, that would blast the roots right out of the substrate. Unless of course you opt for a spray bar.
Cheers!
