Hi VBF, welcome to this hobby,
(Other reefers, correct me if I m wrong)
Since you intended to use the bottom tank as a sump tank, you do not need the canister filter. A compartment in ur sump tank can be designed to take the role of the canister filter already. check out some of the reefer's personal tank threads on how and what they put in their sumps, you will get a better idea.
Regarding tank thickness, dun worry much since the length of your tank is not that long, so the pressure at the centre of the length wun be sufficient to rupture it. Of course, be gentle when handling rocks.
Since you are cycling your waters with liverocks now, and have added bacteria culture to speed up the process, thats good enuf already. Do not add mkt prawns in to rot later, you either add in now, or dun at all. adding in at a later stage to rot will only slow the cycling down.
Chiller-wise, if you dun intend to install one, just get a fan to bring down the temperature. A fan will be sufficient for your tank size to keep temperatures between 27-29 degree celsius. You can still have corals.
As a starter, just stick to the lower-level corals like mushrooms, zoanthids, sun corals, star polyps.
yep, sis monki is right, Singapore weather beri hot. You can consider chiller at a later stage ya? For the time being, just focus on cycling ur waters and testing them with test-kits. One point to note for using fan is that you just have to top up little amt of water daily to counter for the evaporation rate.
Lastly, lights. If possible, T5HO at minimum, if u wanna keep better corals. of course, if budget is a constraint, then a grade lower like T5NO should not pose much of a problem for certain more hardy and les light-demanding corals.