The biggest issue in any airplane purchase is first determining your mission, then looking for an airplane that fits that mission. Too many people get the airplane first, then try to make it fit whatever mission they've discovered that they have. I know people who have 6 passenger retractable airplanes who rarely if ever have more than one other person and a little baggage in their airplanes and never travel more than 300 miles, yet they're paying a premium in fuel, insurance premiums, and in maintenance expenses for that bigger, faster airplane, all so that they might one day fly across the country with 6 aboard.
The next biggest issue is being able to afford it. Airplanes are luxuries--few of us "need" an airplane. Like Hammer said, most of an airplane's life is spent sitting. My personal recommendation is to have ready access to at least 150% of the purchase price. That may be in the form of ready cash or a line of credit, but it's a fact that sometimes very expensive surprises happen to airplane owners. Anyone who goes into an airplane purchase with the idea that if everything works right, they can make the monthly payments on a loan just for the purchase price, isn't being realistic. Insurance can be pretty expensive, especially for a relatively low time pilot, and especially for a seaplane (the biggest reason that few seaplanes are for rent). So all that needs to be checked out in advance. Hangars can be expensive, and sometimes not available nearby at all--and any fabric airplane really ought to be hangared.
Those are the two biggest issues that need to be addressed before any airplane is purchased.
But let me say that there's a lot of unmeasurable pleasure in being an airplane owner, too. Being able to fly my airplane when I want to, equip it the way I want it equipped, and not worry that someone else has dinged it, is well worth it.
Cary