Looking good Harry! I bet you're excited!!
What I would do now is measure the width of the inside of the cabinet for your sump and plan the design. Decide what layout you want, where the down pipe will enter and where the return pump will be. Have a look at refugiums and also deep sand beds as these are features you might want to incorporate for their respective benefits.
To play it safe and as tempting as it may be, I wouldn't put your sump on that shelf. The weight of it would be quite substantial and all that's supporting it is a few dowels. Moisture build up/splashes could also soften the chipboard over time and the weight of the sump alone may very well cause it to sag (happened in my juwel 180 cabinet), so putting it on the bottom of the cabinet would be a much better option. Line the bottom of the cabinet with some polystyrene sheet first too. I would also recommend that you place a fifth foot in the middle of the cabinet base. Again, this will prevent it from sagging and is well worth doing.
It may be a good idea to get some small cabinet brackets and reposition that shelf higher up in the cabinet. This will give you much more room underneath as you'll require space to fit all your equipment and be able to remove it for maintenance/cleaning/repair etc. You can also fix a light source to the underside of the shelf and have it high enough so that splashes are not an issue. This will allow you to grow macro algae in the sump which feed on nitrates and phosphates. You could also fit the extension lead to the back support panel of the cabinet which will help keep your cables tidy and ensure that all plugs for the equipment will reach. Sometimes companies aren't that generous with the power cords on the equipment.
You can also cut two holes in the shelf and fit two 80mm PC fans to it to improve the airflow. It completely removed the moisture build up in my cabinet. It might be an idea too to fit fans to the sides of the cabinet to help pull in drier air. I use a 72mm holesaw for fitting mine and fix them in place with the metal fan grills on the opposite site. To power these, simply wire them up to a 12v adapter. One with a voltage selector is best as you can run the fans at a lower voltage, thereby making them silent yet still moving a decent amount of air. You can use plain black ones if you want the look to be lowkey, or jazz it up with blue LED ones.
As for the internal filter, I'd remove it from the main tank as its a bit of an eyesore. Personally, I hate equipment being on display and always do my best to conceal it.
Before you put any sand in the tank, place a layer of eggcrate in the bottom. This will protect the base from the live rock and safeguard against it cracking in the future. Once you have done this, I'd suggest you add some ocean rock to begin the foundation of your rock piles. Then add your sand. Afterwards you can add your salt water and finally your live rock. Doing it this way means that the ocean rock is touching the base of the tank (egg crate) and isn't going to move, which will prevent a rock slide.
The sand then holds the base rock in place. It also means that when you add your live rock on top of your base rock, it will be fully exposed to water movement, and not partly wedged into the sand. The ocean rock will become live eventually too as the bacteria and critters colonize it. As your sand will already in place, your water will also not be anywhere near as cloudy, provided that you add it slowly. If you pre-mix your saltwater by about two days, then it should be crystal clear when you pour it in to the tank. Fresh salt water tends to be cloudy for a day or so, until the salt is fully dissolved.
That should give you enough to be getting on with for now lol. Good luck