Hi there. You're gonna want to pore over this thread link and the one below it.
Stage zero is pretty imperative, especially if nobody maintained the car in the last few years. Prepare yourself. It's not pretty and your wallet will weep. It may throw up first and then weep, or maybe it's the other way around.
The other thing you may want to treat yourself to is a VIDA/DiCE setup so you can read all the modules and see if there are active codes. It's invaluable to understand what your car is telling you.
Welcome to the R Forum! Despite it's name, this forum is mainly a resource for the P2 S60R / V70R 2003-2007 models. In this forum you will find a lot of information and resources. To get you started. Here are a few links to get you rolling. Swedespeed & R Forum Info for Members SS Forum...
www.swedespeed.com
Stage zero is well worth it. I just had the latter half of Stage 1 done on an 02 v70XC with 131K and it's a absolute dream to drive now: Fuel & air filter, transmission flush, antifreeze flush, belts, TB and tensioner, exhaust hangers, PCV and hoses, steering u-joint and now it's 100 times better than a gently used Honda with a car payment. I'd already done an oil & filter change, new battery, alternator and plugs and coils, torque mounts, among other things like tires and a full alignment, brakes and rotors on all four corners a while back.