hi
lots of questions.
1. your tank is very high and not very wide. which makes it hard for light to reach to the bottom with only 150 watt M/H, also 300mm wide will look very narrow once saltwater is in it ( saltwater makes the back look closer, most salties would not run a tank this size, or you will upgrade / change in the near future (something salties do all the time ))
2. skip the canister filter as it is not needed, ok some use them but with no real benefits to the system.
3. do not use sand / shells / oyster shells you find on the beach. if you want to use sand (i would advise against it until you get more knowledge on how to keep it clean - and dirty it gets very fast)
4. reverse gravel beds belong into the fresh water section, all what would happen is your sandbed cloggs up and your nitrates and phospates explode.
5. i would not aim for any stagnent water anywhere in the system as it will / could lead to problems.
6. growing algae in a sump / fuge sounds easier then it actually is. yes it would need light and then best for 24 hours a day so your algae doesn't crash during reproduction. again i would leave it until a later date if ever.
7. don't make your system to complicated!!!!!
i would recommend for a newbee to start of with a tank (bigger is more stable) which is not to high (600mm max) but at least 500mm wide, a good skimmer (not air stone driven), good water flow thru out the tank, live rock in the main tank, no sand at least in the beginning (yes i do have sand and would not go bare bottom but it is a lot easier to look after for someone new to the hobby).