Just got a 98, have some questions?

[98GTNewbie]

Hello to all. I am hoping some here can answer some of my intial questions. My car which I just got 3 days ago. Is a 1998 GT and looks like new inside and out. It had one issue when I purchased it which is going to be fixed Wednesday. The car has 77000 miles on it. The rear end needs new bearings and one side needs an axle shaft. They are also going to re-do the wheel bearings. The engine sounds solid and the car handles very nicely. Now on to my questions. I would like to do some minor upgrades. I need to know if anyone has experience with the K&N Filter and how much HP and Torque that will add. As well I was considering a underpulley set, but read somewhere about a person doing this and then having issues with his check engine light as well as alternator and battery? Is there a set that is fullproof, or close to it. Also once again how much HP and Torque will pulleys add. I will need to have a mechanic do most upgrades so that is going to add to my cost. Any recomendations in Las Vegas? One last thing can this engine / miles handle these kind of upgrades? I have read that the two I mentioned are simple and actually can help the engine work easier?

Thanks for any info, Chris