^^^^^^^^^^^^^^^^^^^^^^^^^ what he said.
Down pipe and a front PLOG will be the cheapest solution to exhaust flow, and if it's going to be a year before you start any modding, consider those "preventative" maintenance as well. They'll help keep the engine safer by increasing the exhaust flow and keeping heat away from the engine, thus reducing the chance for detonation/knock and possibly eliminating what KR you might already have.
Also, definitely save up your pennies and get a tuner, or even just a wideband. Those are the two biggest, most important "mods" that will keep that car on the road, regardless of how much you throw at it. They will save you money in the long run.
Unless the suspension was dealt with by the previous owner, you might want to check it out. If it's all worn out, you can definitely upgrade alot of it for pretty cheap, thank GM for making a police W-Body. Consider the upgrades, an enhancement of safety. lol
Check out the W-Body tech page on Overkill Motorsports, Will has a subsantial amount of info over there, alot of which is very applicable to your car.
