use the auto-rx treatment on your engine
www.auto-rx.com
than use about two big bottles of LUCAS FUEL INJECTION CLEANER (one bottle is enough for 4 tanks of gas (8 ounces each gastank) you can buy them at the local car parts store. I used to do it like this. before you fill up your car, pour 8 ounces into the tank. use this at every fill-up you make for at lease the first 8 fill-ups and after that at least every 3-4 fill-ups get 8 ounces of it in there. This helped me keep my caravan alive, and also helped me regulate the rpm on my passat.
Clean the **** out of it. what i did when i got the passat was:
I removed the seats. deepcleaned the carpeting and upholstry. rented a carpet cleaning machine for that. this is for two reasons. one beeing thet you want a nice clean car, now don't you... the other beeing, probably the car has not been cleaned like that before. even if it looks nice, it could be bad. like you never know who owned it, and all kinds of bacteria and a lot of disgusting things could be in the upholstry, and carpeting. so it's healthy to clean the car out deeply. i also deepcleaned the seats. I also used some of my wifes dishwashing solution (in this case Palmolive) to scrub the plastic parts/dash/steeringwheel. you wouldn't believ what the machine, and the sopay water looked liek after the cleaning. so at this stage you have a clean car. If you are lucky like i was, you gonna smell the factory scent of the car after a couple of days. now you should apply some armor all (also found at local parts stores) to the plastic parts. all of them. you better do this until you still have the seats out, so you can reach all the parts of the car. oh. and when deepcleaning, don't forget about your headliner. i almost forgot about it.
as for extrior, get a good thurow wash, rather was by hand with a car-washing brush (walmart $2.50) and clean water. no shampoo is the best or only use good quality shampoo. if you want to go all the way, (i stil have to do this) polish the car. don't wax it, polish it. i use mothers 3 step system. three different polishes.
1. cleaner: cleans all the old waxes, polishes, tar and road debries off of your car.
2. Polisher: it polishes the **** out of your car. By this time you would blind yourself if you look into the polished areas. it's that good.
3. sealer: as it sas. seals the new polish and protects it. (all three cost like $9)
now all the three steps only have to be done once or maybe twice a year. after this you would need to re polish and seal it about every 3-4 months.
some tips with polishing: only polish above 65F outside, because under that you would harm the clearcoat. also only polish in shade. you can screw up the paint if you polish in sunlight.
Next: have the engine shampood. you better go to a professional shop and let them do it, it's about $60-$65 and you would do it once a year. so it's grease free and nice looking.
check and replace all of the fluids. you already replaced the coolant which is good, but you also should flush it. it gets all the louse debrees in the system out. to flush it it's about $20 at the local shop plus the fluid. flush the power steering, and brake systems too. it's not a life-death necesity to do these but it gives a trumendous amount of peace of mind tat you have a clean car. if you can, change to synthetic oil. it's more expensive but it's worth it. let the local shop check the turbo out. you may have leaks that prevent it from functioning properly. and again auto-rx the engine. you can find more info on the link above. i'll post a link later from someone that tried it, and documented the effects. check all the rubber seals arount the trunk lid(s) doors.
Oh boy i think i crapped a post like an elephant. this is what i would do to a car when i buy it. even if it's newer. i will continue doing all of that on my car too. with time though. o one more thing. check the brakes, and if necessery (or maybe even if not) change the rotors and pads. you gotta stop after going fast.
sorry for the huge amount of info here. hope it helps. i still come up with new and new ideas but this post is already way too long. I may do a sticky thread on how to keep the car healthy.
cheers
Good luck trying to drive 2 of your mates around 
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