Welcome to the world of goldfish. I've been there for a long time and don't plan to leave any time soon lol!
I would keep the number of fish at 3 or 4 in the tank. Fancy goldfish get pretty large. They aren't as long as the pond goldfish but they maintain a lot of the bulk. With just HOB's you should probably stick to 3 goldfish.
You most certainly don't need a chiller. Goldfish aren't a true coldwater fish like trout. They can easily survive temps of above 80F. Mine do just fine in the summer when it gets 80F. Just make sure you feed them more often at higher temperatures as their metabolism increases in warmer water.
A heater is personal preference. I have one and only set it on 72 in the winter because I don't like sticking my hands in freezing cold water. My house is set to 65F, and the water gets pretty chilly otherwise. So it's really up to you.
As to substrate, I prefer a sand bottom tank. Some people like barebottom because it's easier to clean, but seeing as my goldfish spend like 70% of their time sifting through the sand, I don't think its right to deprive them of that. But sand and HOB filters don't exactly mix in my experience. It's going to come down to what you want to do.
Cleaning sand isn't difficult at all. Just hover the siphon a couple of inches above the substrate and most of the stuff comes up. Unlike with gravel where the crap tends to get caught in the spaces between the particles, there is so little space with sand that not a lot of crap gets down in it.
As Flint mentioned before, nutrition is a key part of fancy goldfish keeping. Quality food is super important because their digestive tracts are morphed. For a pellet food I like New Life Spectrum. My goldfish get their Garlic Guard formula as well as Repashy gel foods. Frozen foods are also great for young goldfish as they need a lot more protein than older fish.