The problem with sorcing this kind of thing on line is that the tanks are usually glass or acrylic and not over 10g. What you need to do is either compare the kits you are looking at (sans what they are missing) with the cost of a tank, hood and stand you find, along with the accessories I find for you. Which is better/cheaper. It may be better for you to buy a kit and add on what is missing.
The kits you've seen sound okay, can you tell me what the filter is and how many gallons its rated for? It might be better in the long run to double filter via redundancy. Meaning that you get TWO filters rated for 50-55 gallons instead of 1 for 100-110 gallons. That way if one fails, you'll have a back up and you won't have your fish totally wallowing in waste. This is what I'm doing with my son's tank, anyhow.
This stuff is all at BigAlsOnline.com:
Filters:
Penguin Bio Wheel 200, rated for up to 50 gallons -- $20.99
Pengiun Bio Wheel 350, rated for up to 70 gallons - 26.99
Aquarium Systems Visi-Therm 200 Watt Heater - 16.99
I added the thermometer since you indicated that you were considering more tropical fishes.
Gravel I would buy locally as it is usually charged as overweight online and it works out to the same price pretty much. You may also want to consider not having gravel at all. With goldfish it's easier to keep the tank clean when there is no gravel. Depends on what you want
Now, what I'm buying for my son (I'm getting him a 45 gallon for his two goldfish) are two Penguin 200s. Double filteration + redunancy. So that's 42$ plus shipping -- say $50.
If you opt to buy a kit and the filter is a decent one with adequate filteration for a 50 gallon, then just buy another filter and add that on.
Hope this helps,
Roan