Hey there and welcome back to the hobby!
A sump is preferable for a number of reasons. It gives you a bit more water volume, making the system more stable. It also provides a place to hide away a lot of your equipment, making the tank itself more attractive. However, by the sounds of it, you already have a tank, and an undrilled one at that. You can get the tank drilled, to enable you to fit an overflow, but if that sounds like too much hassle/stress, it is completely possible to have a tank without a sump. The skimmer will need to be a HOB (hang on back), which the Aqua C Remora is, so you're fine there. If you don't want to drill, but want a sump, it's also possible to have an overflow that fits on the side of the tank, working on a siphon basis, but that worries some people (what happens if the siphon fails?).
As for your other questions (lighting, how much live rock, how deep a sand bed etc), it'd be useful to know what you'd like to keep before giving you advice. Do you want a FOWLR (fish only with live rock) tank or a tank with coral? If you want coral, what sort of coral are you most interested in keeping? It'll make the world of difference if you want to keep soft corals or LPS (e.g. Acanthastrea sp., Eupyhllia sp. like Frogspawn coral etc) or the rather more challenging SPS (e.g Acropora sp, Montipora sp). If you eventually want to keep an anemone (for which you should wait at least 6 months after the tank is set up), then that will also have an impact on how powerful the lights should be, even if otherwise all you want to keep are soft corals. If you want a mix of everything (starting with soft and building up eventually to the more difficult SPS), then it's best to plan now with that in mind.
Even if you don't know for sure, give us some initial thoughts and we'll try to guide you to what makes most sense.