APS are pretty good for the money. I don't have one, but from what I've read they are more fragile than the more expensive brands but if you are aware of that and be careful with it, especially while cleaning it and putting it back together, there should be no problems.
I'm not familiar with the brand so can't offer much of a detailed opinion on it. I would make sure it has some good reviews from people using it for saltwater since some filters can be destroyed faster by saltwater if they have water-exposed screws etc. that can corrode. I currently use fluval canisters and also used the API ones in the past.
Just remember that you want to configure a marine canister filter like you're doing an enclosed sump substitute. It's often not as plug and play as with freshwater. Ideally you will get a combination of colonized media and tiny crustaceans (mostly Amphipods) living in there that help break down waste to minimize the amount of cleaning you have to do in the long run.
If it comes with ceramic rings, you can use those or use live rock rubble in place of it (the LR rubble boosts filtration faster since the ceramics have to colonize). The sponges can be kept if checked regularly and cleaned thoroughly when needed or they can be substituted for other things depending on how much space the filter offers. If the filter comes with ammonia remover media, do not use that - it's typically a freshwater-only media.
You will most likely want to run both activated carbon and then either granular ferrous oxide (GFO, reddish brown granules) or the green aluminum-based phosphate remover pads. GFO is more effective but the pads can be easier to use.
30cm seems small to me for a sump; once the return pump and partitions are in there, it doesn't seem like it would offer much over a canister in terms of filtration boost. A lot of standard sump stuff like filtration socks would be cramped in there as well. However, before even looking at what to use as a sump, you should determine if you can even get a reasonable hang-on overflow for your tank and/or if you want to deal with the challenges associated with one (possibility of siphon break, etc., and this is of course assuming your tank isn't pre-drilled to accommodate horizontal pipes directly at the waterline). I have never seen a hang-on overflow I would be happy with on a smaller tank and personally would never want one which is why I went with canisters. I had an overflow once on a larger tank once and even that was a decent amount of hassle to ensure that things kept running smoothly.
I went to my LFS today and talked to some of the employes and I've decided to do it right and buy a proper saltwater tank with the light, and filters and everything in it.
I'm going to sell my current freshwater tank to fund it.
All I need to buy else is a heater and a rodi water system and the rock, sand,fish and coral.
If I'm missing anything or if u have any advice for a beginner marine tank keeper then I would greatly appreciate. Thanks
Even a 60 litre tank is small for marine standards. An aquarium that is 3 foot or bigger is a nice starting point if you have the space and finances. It doesn't cost much more but gives you more water volume so things remain more stable.
If your current heater is big enough, you can use that in the marine tank.
You don't need a special marine light unless you plan on keeping corals. Even then you only need globes with a 6500K (K is for Kelvin) rating. You don't need ultraviolet or actinic lights for marines.
You will need a hydrometer. They come in a floating glass type, plastic chamber and refractometer. The glass is cheapest but not that easy to read and sometimes break. The plastic chamber is good, easy to read and not too expensive. The refractometer is the most accurate but costs a bit more than the other types. however, they are not much more expensive than plastic chamber hydrometers now.
If you are using artificial marine salts, you will need a couple of big buckets to make up the salt water before you use it. The marine salts need to be mixed with the water for 24 hours before use so all the salts are dissolved. This salt water should be aerated to make sure it is thoroughly mixed. You use the hydrometer to test the salinity (salt level) before adding the water to an aquarium with livestock in.
Marine tanks need a coverglass to stop evaporation and reduce salt creep.