Welcome to Tropical Fish Keeping forum.
Most of us unfortunately buy our first fish tank and then after we have the usual initial problems and dead fish go looking for help. It is great that you are doing things properly, and finding out beforehand.
To your ideas. Goldfish (Moors are "goldfish" per say) do not mix with tropical fish. Temperature is the main thing, but there are other issues. So your first decision will have to be whether you want a room temperature goldfish aquarium or a tropical fish aquarium. Either will need the correct type of filter [this varies too], and a light for observation and perhaps live plants; tropicals will likely require a heater unless you are in a year-round warm climate.
Tank size as you have mentioned is dependent upon the fish you want to keep. I will say that goldfish need larger tanks than most realize; they produce a lot of waste and water quality can very quickly deteriorate. The larger the tank, the more stable will be the water, this holds for tropicals too. And most of us think we will have a tank with this and that, but before long we wish we had space for more fish. So once you've decided on goldfish/tropicals, then consider the largest tank you have space for and can afford.
I'll end by pointing you to our fish and plant profiles, under the second tab from the left in the blue bar across the top. Most of the commonly-available tropical fish are included, categorized according to family or order. Clicking on the heading of each section will give you a synopsis of that family, and then the species are listed below the heading. If the common or scientific name is used in a post exactly as it appears in the profile, it will shade and you can click that to see that fish's profile, as Goldfish, Corydoras aeneus, etc.