OK, I see a few problems with your design.
1. You need to lower the pump and enlarge the Return Chamber (the section with the pump). A sump system is a great asset to any system. One of the greatest benefits is that the water level in the display aquarium never drops due to evaporation. Instead, the evap will show in the return chamber. The return chamber needs to house enough water to cover the pump at all times, plus enough water to cover evap for at least 24 hours. Keep in mind that your evaporation rate will increase with the use of a sump. Basically, I would recommend that your return section house at least 4 gallons of water.
2. Make sure you have enough extra room in the sump to cover power outages. In the event of a power outage, water from the display will drain out to the lowest point allowable by the overflow. The sump will need to house the afforementioned contents on the tank, plus the contents of any plumbing between the two, and possibly any other equipment that might drain out in the event of a power outage. I'd recommend your baffles be no higher than 5"-6" below the top of the tank
3. You may have a problem running a HOB filter in the sump while allowing the necessary room for the requirements listed in #2 of these recommendations. I would seek to incorperate the filtration in the design of the sump rather than adding the HOB filter.
4. Since a sump is gravity fed, It needs to be housed at a level below the tank. The most common location is in the stand below the tank. Since Canister filters also need to be housed below the tank in which they are installed, you may see an issue with installing the Eheim in the sump. Again, I would seek to incorperate the filtration in the design of the sump rather than adding the canister filter.
5. There are things that you will need to consider for the proper installation of the plumbing. We can discuss this later after a design has been established that is both properly functional, and suitable to your specific needs.
These are just a few of the issues i see. I would highly recommend that you review the questions and responses posted in the "Sumps and Refugiums" section of "Starting and Maintaining a Saltwater Aquarium". This will help you to gain a better understanding of the functional needs and design standards involved in the use and design of a sump. I would also invite you to take a look at the sump I designed and constructed for use in my reef. This 30g long design has been in operation on a 55 gallon reef since november of 2007 without failure. I would be more than happy to provide you with additional assistance in the design and construction of your sump should you so desire.