Hello!
I'm not sure about filters but i have a fluval 404 (very large and sits out of the water). I've always had Fluvals - they're really good value for money, easy to maintain and run forever! very cheap to buy the media and sponges as well.
Heater - I dont know what brands to look for but I always go for one that has a precise temperature guage and make sure its easily read and easily adjustable from the top of the water.
I would err on the side of caution with seahorses. I think they require high quality water which would mean getting a protein
skimmer - you're talking a lot of money as well as a
sump tank underneath whcih means drilling holes for tubes and extra pumps for pushing the water back up etc. I've been told they need a minimum of 55 gallon but the dwarf species could probably be in less.
I think 2 clown fish in a 20 gallon is a bit too small - see link
http://www.fishlore.com/Profiles-ClownFish.htm - says they need 30 gallons each. Maybe try a little goby ? I'm not too sure what else could go in something so small... sorry!
Definitely get some
live rock - its a great filter and very pretty too : )
Good luck with your marine starter - do all the reading you possibly can before getting it - it's a very costly hobby! I personally would reccommend saving up a load of money and buying a ready set up tank with all the fish and sand etc - it works out A LOT cheaper and makes life a lot easier too!
Urs