Hi all
New member here \o/
I recently became the proud owner of a 2005 Mk5 Golf GTI. Had 124800km on the clock when I bought it. Needs some work here and there, but that's where the fun starts right? 
First things first will be a proper service and replacing all the parts that commonly fail on the Mk5's. After that there's a bit of nice to have's I need to get sorted. And after that hopefully some tuning...
Hey man congratulations!
I still remember how I felt the first day I got my Gti.
Moving from a normal golf and Passat to a Gti, it was unforgettable.
We all do know it's not a super fast machine like AMG or M3 or 911 turbo, but it is joyful.
I think the reason is similar to why people like MX-5. It's just fun and satisfying.
You might already know this but what I am suggesting are these:
1. change the cam follower ASAP.
the dealer usually wouldn't replace it. There are tons of videos on YouTube.
I recommend you to buy one from TradeMe.
2. So when your car is about 120K, it's time to check spark plugs.
stock mk5 gti usually use expensive NGK Platinum spark plug which would at least last about 8k~120k.
if your car feels misfiring, it might be the spark plug or the ignition coil.
I'm now using NGK iridium spark plugs, which is much cheaper (1/3 price) but last shorter, and no performance difference.
3. ignition coil. they are expensive. a whole set costs at least more than $270.
but try not to replace them for no reason. it won't make your car run smoother.
a mechanic has told me that you might need to replace just the faulty one only when the CEL is on.
4. if your car has rough idling, check the PCV Valve. I used to use Aluminum plate PCV delete kit but I swapped back to OEM.
The reason was that it made terrible emission. Black smokes came out regularly.
After changed back to OEM, no black smokes.
5. Diverter valve. Yes it is very important.
there are several version of them. They call it revision C, D, G.....
C is the worst one. D is piston type which is the best one.
if you hear some whining noise, it would be the DV valve.