I just purchased a 2001 Merc 225 EFI w/ 500 hrs on it, It hasnt been used much for the last 3 years. Before i start fishing, i am wanting to preform a complete tune up. I was thinking of replacing the lower unit oil, replacing the fuel filter, and replacing the plugs and ignition cables. Is there anything you can think of that i am forgetting? (ie other filters i am not thinking about, certain gaskets i should be checking, etc)

I do know the impeller was changed this spring.

Any tips or instruction would be most helpful!!

Check/clean ALL ground connections to the motor, tilt/trim, ignition system. Pull tilt trim relays and clean sockets with electrical contact cleaner. Grease pivot points on motor and tilt assembly (inspect lift ram but DO NOT GREASE IT). Inspect any and all fuel hoses for cracks, leaks, collapsing. 500 hrs would be a good time to retorque your mounting bolts...if memory serves it should be 105 ft/lbs but don't hold me to that. Take a ball peen and lightly tap across your upper and lower transom...listen for any dead spots and inspect for cracks in the gel coat (especially in the lower transom area). Pressure test the lower unit if possible. If it has been sitting most of those 3 years, it'd probably be advisable to completely empty the contents of your oil canister and refill with fresh oil. Compression test the cylinders if possible as well. Inspect/tighten/clean all connections to your starter solenoid. Oh and above all else, if you put it on muffs in the driveway, resist the urge to hammer on it...lower units were designed to rotate under load and excessive rotation without load can damage them. 12-1500rpm maximum if you must.

LOL...that's all I can think of right this second

Change out water pump.