I like the 40g better just because of the bigger foot print. You can fit more into a 55g though.
First you need a overflow. Look into the different types of overflows available to you and everyone will tell you getting your tank drilled is better than using a HOB
overflow. The number one complaint about overflow is the noise from the overflow area and the noise level from the drain lines. You do the research and pick the one you like the best.
Each overflow and drain line will allow a maximum gph
to from display tank to the sump. Knowing the maximum will help you choose your return pump. If a certain overflow allows a maximum 800gph, then your return pump must not return more than 800gph. Return pumps are affected by gravity (height from pump to the output of the display tank), number of elbows you use, and the diameter of the return line. There is an online calculator that can help you with these calculations, PM me if you want these calculators.
Sumps are made in many ways. If you just want basic setup, then most sumps are built around the skimmer. Overflow drains water to one side of the sump, flows through the other side, and pumps water back up to the display tank. Baffles are used to help minimize annoying microbubbles (look this up if you don't know what I'm talking about). The skimmer would be furthest away from the return pump to help minimize microbubbles. Put in a refugium if possible to grow macroalgae like chaeto. These are 2 of the most basic things you can put into saltwater tank that will benefit you the most. The flow is better if its slow going through the sump. The faster the flow, the higher the chance of microbubbles getting to your display tank.
skimmer is definitely not as effective as sump style skimmer (ok either in-sump or externally hooked sump). Reef Octopus is a good pump for the money, but the big ticket item for any skimmer is its pump. Many Reef Octopus pumps have decent pumps (some have good pumps), but they still perform well for the money.
Better to do a quick google search to get more faster answers to simple questions and post when you have specific questions.