Fixing everything on a car?

i have a 92 nissan 300zx non turbo automatic. had it 4 about 2 months now and it has now 140xxx miles on it. when i 1st got it everything worked fine. the mechanic who checked it out b4 i bought it said the transmission shifts late but its nothing big. about 1 month after having it i wanted 2 see what it could really do so at a light when it turned green i put it on overdrive and floored the gas. after it started making a ticking sound from somewhere around the engine. ive been told it could be alot of things but im not a car person so i dont know. i had both catalytic convertors changed. i am going 2 buy the best parts 4 my car b4 i bring it in 4 a major tune up (what parts do you guys suggest. sparkplugs and stuff.) now when i start the car it squeaks and then stops. would it be ok 2 just bring my car into a shop have them do a tune up and say "i dont know whats wrong with my car find out and fix it"? money is not an issue time is since this car is a keeper. i want it 2 run perfect