Dude, get a cat, it's much more fun.
I've had goldfish for the past 13 years in my home, it's absolutely shit, they kill any other fish you put in there and they shit like crazy motherfuckers so you need some robust snails as they kill the cleaner fish.
On the tank size, are you planning to put it next to a wall or in the middle of a room? If next to a wall, go small, 40 gall is more than enough. if however in the middle of a room, get as big as you can so the fish can have as much fun as possible.
Still, don't buy goldfish, they're mean motherfuckers who only desire to kill other fish. Neons and those fancy fishies are great, put those in with a few bottomfeeders and glass snails and you're good. Don't forget the treasure castles in the aquarium as well.
When buying the tech, ask for things you can easily maintain yourself (i.e. change the sponges in the water filter), if you go too pro you need to call a technician every time or bend all kinds of wires into weird shapes so you could get hold of components meant to be taken out with pro tools). Do not go dirt cheap, try to stay in the mid range.
Regarding thermometers, unless you put the aquarium in the sunlight (which is a very very very very bad idea, your fish will want to kill you) , a simple stick-on works fine, they go for like 2 dollars.
As you want to keep the aquarium pretty cool, (your room temp is around 78f im guessing) you might need multiple pumps and some small fans, in the summertime we had to change the water two to four times every week because it got too hot, we had a 30 gal tank with one pump.
Again, goldfish are shit. they take 15 gall each and kill everything.
