Hi Dragut,
Welcome back to the hobby. I am a similar returnee after keeping fancy goldfish in what I would now describe as a tiny tank. When you say the hobby is far more scientific now I think that may be just down to good information being more readily accessible. I bet even 30 years ago there were some stellar fishkeepers who understood the importance of the nitrogen cycle, etc. Only 5 years ago I thought I was doing a great job scrubbing out the tank once a month and washing the filter in untreated tap water. After reading just a few articles on the web I would now never dream of doing such a thing!
Given that you've taken the time to read up on fishkeeping before buying your fish is a good indicator that you're sensible and likely to be a successful fishkeeper. When I was restarting the hobby, the best bit of advice I read was "If you look after the water, the fish will look after themselves". Whilst this isn't 100% foolproof, its a great start. Angelfish prefer softer acidic water, so I'd check your water to see that it is favourable. You know that fishless cycling is the way to go and it sounds like you're inheriting an establish filter. From a purely cycling point of view, I think its best to keep as much filter media and substrate from the existing tank as possible to help maintain the bacteria. Sanjo is right, goldfish are incredibly messy fish, but this should mean the bacterial colony is well established. I'd do an almost complete water change and try to vacuum as much crud out of the substrate as possible. Scrub the inside of the glass if necessary, but don't use any chemicals or cleaning apparatus (sponges, brushes, etc) that have been used before. Unless you're feeding the tank ammonia, don't leave it too long before adding a couple of fish as the bacteria will die back. If you don't fancy adding ammonia, I would start with two angelfish for the first month and then add another one or two every month over a 6 month period in order for the tank to fully establish.
My only concern is that there may be diseases within the existing tank, however if the goldfish are healthy then I wouldn't be too concerned with this. It seems to me that there are so many ways of introducing disease to the tank accidentally its just part and parcel of fishkeeping.
Now angelfish prefer taller aquariums and the thinkfish profile recommends a length of at least 90cm. With a 100ltr tank with an internal filter, that equates to 10 angelfish maximum. If you want to keep other fish types you'll have to reduce the number of angels. Small (non-fin nipping) tetra would do well with angels. Neons, cardinals and rummynose spring to mind, but there's a whole host of possibilities. Cories would also work well. Use the community creator and find your best combination.
I'd also plant the tank heavily as angels love vegetation. I always think vallisneria works well with angelfish, but that's a personal preference.
I'm sure you'll get plenty more opinions here soon, but good luck with it all!