I'm in a similar situation myself with my intentions of going into marine.
If the 60cm tank is the Red Sea E170 it does kinda look nice and has all the bits you need including the filtration system and light. However at nearly £1300 it makes me wince. My current 120 ltr tank is 80cm wide however the total capacity is less than the E170 at 170 ltrs but that drops to a smaller capacity with the filtration system taking up space.
Your 100 ltr tank would similarly drop to a useable capacity of possibly 75ltrs. However I don't agree that it's too small. I have seen some remarkable tanks of much smaller size that make freshawter tanks look so damn dreary.
I have the All Pond Solutions filter, it's actually a SunSun. I have 3 of them. They are as good as any others, and in fact better than another I had which leaked and couldn't get a seal for. The only problem I have is that one of them hag the lifting lever snap when I flipped it over the disconnect the hoses to clean the filter. Not fully though so I can still use it.
My LFS that has a good number of marine tanks uses canister filters although they use two per tank, which however are at least 250 ltrs capacity.
I have received comments on my own topic that I don't really need filters if I have a good amount of live rock and sand as the biological chemistry is ongoing in a much bigger way in the actual tank. To that end I'm going to empty my 60 ltr feshwater tank and make it into a factory for live rock and sand so that when I'm ready I can just drop the rock and sand into my 120ltr tank without the need to go into a cycling period. hopefully at least. I don't want to have an almost empty tank in my study for months while the marine tank matures enough to use it.
I live by the coast in an area that's got limestone on the cliffs so there should be some chance of scooping out sufficient rock and sand if I can get my car close enough to the beach. I have a totally knackered back right now so I may have to delay this until my back improves.
Until a couple of decades ago, all that was showing on the beach was coal slag from the mines (Easington - as shown in Billy Elliot) but now the area has changed so much to make the beaches some of the best in the UK, albeit no-one thought of putting heaters in it to warm the water for swimmers.
All that aside, my intentions are to stick with the 120 ltr freshwater tank with all of fish in it until the collected rock and sand is active enough. I will be building an internal sump filtration system just for peace of mind then slowly add bits and pieces of corals and suchlike to get it going. There are plenty of Youtube videos showing how to build a filtration system. And the cost of buying a complete new tank system is gone. Basic costs only for the sump materials, light etc.
