Impossible to evaluate the value of a 2002 Parker 2520 without seeing the condition of the boat, engine , electronics etc. Could be in great shape or a train wreck. Aside from the engine, another major area of attention on a boat of that age is the fuel tank. At 18 years of age, the fuel tank in that boat could still be OK but you are absolutely in the zone of concern. Unfortunately the aluminum does tend to corrode over the years. Even if the tank is OK, at that age both fill and vent hoses are well into the age they can be expected to have issue. The gasket at the sending unit is also an area of concern.
Filling the tank with fuel till it vents and then removing the tank inspection plates and getting on your hands and knees and sniffing for gasoline fumes doesn’t prove that you do not have an impending fuel issue if you don’t smell fumes but if you do smell strong fuel vapor, you can assume that fuel is leaking somewhere in the fuel system and needs attention. Replacing hoses and gaskets isn’t a huge expense. Replacing the tank is a much bigger deal, unless you do all the work yourself, the job will cost in the vicinity of $6000.
Not saying not to buy a 2002, could be a great boat with many years of service ahead. Fuel tank issues are fixable, just figured I’d toss that info out there in case you were not aware of potential fuel tank issues with a boat of that age.
Best of luck.......