I used to keep tropical marines (Nemo & friends!) and had a 800 Litre tank in my living room! Was really sad to let them go as we'd had some for about 4/5years, but when D came along so close to the puppy we had to rehome them all and sell off the tank.
Marines are amazing, but the smaller the tank the harder the work as you have to keep up maintainance really regularly to ensure good water quality for for wet friends.
I'd highly recommended shopping around for 2nd hand equipment via Aquarists Classified (website) or even fish forums. The price of good quality equipment rapidly drops when you go 2nd hand. We bought our huge tank 2nd hand about 3/4 yrs ago for £600 and then sold it on for £300. The guy bought it customade for over £2000!!
Also please don't be fooled by the myth that "A fish will only grow as big as the tank!" I tell people that if that were true we'd have fishes bigger than countries in the ocean and whale sized fish in lakes!!

So make sure the breed of fish is suitable for the tank size.
My advise have a measure up for the space you have, decide whether you'll be getting a cabinet or not and work out the volume of the tank you'll be after. Then you can look at what fish you can get.
A nice style to get is a Nano tank which hides the filtration in the back of the tank, one with rounded edges for Joel to have really good view all the way round too!
Hope you have some great fishy fun together!!