I would vote yes on the custom caddy. The filter socks 2" on nanos are really a pain to wash. Although I am not sold on the fiber-ball thing that comes with the caddy, filter floss or a pad works great. The pump that comes with the IM15 is a downgraded pump so that might be the reason he mentions that. On my fijicube 10g I run a Sicce .5 and that provides a great amount of flow. I am not sure of the relevance of a DC pump in a nano, some ppl. run DC pumps because of the noise/hum with an AC pump. That may not be an issue with you. Also most AC pumps especially Sicce are bulletproof. As far as a skimmer goes. I use skimmers on all my nanos, they are also the eshoops nano. That skimmer is amazing, always great skimmate right out of the box. Some ppl. say that why do you need a skimmer if you are doing weekly water changes? Well for one, it is a great way to improve your PH and two, it is pulling out stuff even with the water changes. For a light, I would go with a AI prime HD, A bit more money, but you might be able to find one used. At least with this light you can use it later if you upgrade, and be able to control it with a AI, Mobius or Apex fusion app. For sand I use the CaribSea Ocean Direct and have had good luck with it. It comes with various grading and even little shells. Hence the name. But I have also used their special grade which I like in a higher flow tank. As far as how much, that depends on if you are going to be putting a sand sifting goby or other sand dwelling creature that you would need a deeper sand bed. If not I like to run a thinner bed, maybe 25 -30 mm <---- see that I am converting it to Canadian for you... lol. One other thing that I don't think has been mentioned is getting a variable speed power head. That is, I feel a must. There are many out there, and because it is a small tank you do not need a lot of power. I use an MP10 on it's lowest setting in tidal mode. Again if you get something like an ecotech MP10, that can go with you to your bigger tank in the future. An ATO is a must because the evaporation makes a significant difference with your salinity. I use a ReefBreeders ATO but I had to cut out a circle of vinyl because it is the laser/prism version. There are many ATO's out there, you just have to find the one that works for you.
Hope this helps.