Depending on the breed of your friend's goldfish its lifespan should be anywhere from 10-40 years.
Hole in the Head disease is a parasitic disease that usually affects fish kept in unsuitable environments, or fish under a lot of stress. It begins with discolouration of an area of the head.
Guppies are a good beginner fish, you can keep about two for every 1.5 gallons, but you should keep less in a tall tank. Tall tanks allow for less oxygen exchange, so be sure to purchase an airstone or two, or even a bubble wand for your tall tank. Live plants will help as well. While guppies are a hardy fish, it is best to cycle your tank before adding them. Here is an article on cycling: http://www.aquaria.info/modules.php?op=modload&name=News&file=article&sid=344&mode=thread&order=0&thold=0
However, if you pack your tank with live plants, you won't need to worry about cycling, just wait a couple of days before adding your fish. Which brings me to another point, you should be adding somewhere around five guppies every two days to avoid a mini-cycle, which can kill your fish. Aim for tall plants like wisteria, amazon swords, anachris, and onion plants in the back and shorter plants, like java fern, glossostigma, and liverwort in the middleground and sides.
If money is an issue, look around your local fish stores, classified ads, and craigslist for used tanks. Avoid anything that has been resealed. Your aim for cost should be about $1 per gallon. At my local fish store they have a 30 gallon hex tank with stand and hood for a reasonable $60. [In a 30 gallon tank you should have around ~20 male guppies]
On to the equipment. You have three kinds of filters to choose from: Hang-On-Back, Internal, and External. For an external filter, you will need holes drilled in your tank by a professional. External filters are good for larger tanks, usually those 55 gallons and up. Internal filters are not very favorable and I wouldn't recommend them. Which brings us to HOB filters. If you get one of these for your tank, you will need one that filters more than what your tank actually holds [if your tank is 30 gallons, it should filter 40 or above.] This will make it easier on everyone. Aquaclears and Whispers are the most common filters, and the best. Whispers are cheaper and easier to maintain. Remember to clean the filter sponge and carbon filter at separate times [change the carbon once a month and clean the sponge once every 6 weeks]. You can purchase an extension for your filter if you wish, but with two sponge filters with airstones, you should be set.
Gravel vacuuming will be unnecessary in a heavily planted tank, so don't bother buying one of these.
Most flourescent lights provide the necessary colors of light for live plants, so no worries there.
Test kits. These are VERY important. You need to test your water for ammonia before adding new fish and you need to test water hardness, pH, KH, nitrites, nitrates, etc regularly. My recommendation is for you to purchase an API ammonia test kit [you will probably never need to replace this] and Quick-Dip 6-in-1 sticks [you'll need to replace this every three months or so when you run out of sticks]. This is the cheapest route.
QT tank. This is unnecessary unless you want to add new fish after your guppies [if some die off and you with to replace them]. Simply treat your new fish with a dose of Melafix [cheap anti-bacterial fish medicine] for a week.
Algae cleaner. This can be useful, but even more useful are otocinclus. They are a very small catfish that live for algae. Don't add them unless you have an abundance of algae in your tank.
Net. You need a net for catching the new fish in their bag and putting them in the new tank [do not add the water from the bag].
Dechlorinator. You need to add this to your initial water and new water that you introduce into the tank. Fish can die quickly of chlorine poisoning.
Heater. Guppies are tropical fish and need a heater. Thermometers are cheap and necessary too [guppies prefer a temp of 75F-80F]
Maintenance for your tank:
Water change anywhere from 20-30% about once a week.
Daily check your fish and make sure that they are all active and eating [my guppies feed from my hand.. it's very cute!] and that you don't see any signs of attack by another tankmate.
Check your temperature daily to ensure that the heater is functioning properly.
Test your water with the 6-in-1 strips weekly.
Monthly change the filter carbon.
Daily check that the filter is functioning properly [put it on maximum flow, I've found that guppies enjoy swimming against the current].
Guppies are an amazing fish, and they compliment a planted tank very well, as they are available in a wide range of colors [I like your gum ball machine Idea too
]. They live 4-6 years if properly cared for.
Check around the articles and FAQ at www.fishgeeks.com
in the future if you're having any problems!
Also, here is an example of a heavily planted tank:
That fish is an arowana, one of my favourites!! They look like grumpy old men