Hi
I was in a similar situation with my 2001 s60 2.0T (175000 miles) about 6 month ago. I was eager to throw performance mods at it, but the more I red about it and watched youtube videos to educate myself I realised how often people mentioned stage 0 tune. I am glad I waited and did a lot of planning before jumping into it. I decided to check/ repair many things on the car myself. Here is the list of things I did so far.
Back in November the excessive oil pressure in the crankcase pushed out my intake cam seal. (engine survived as it literally happened outside my home/ so lucky) So new cam seal and timing belt with tensioner/ idler pulley done. With the help of this forum and the internet I knew the PCV system need attention so did that too including all related hoses/pipes. 5 new Hella ignition coils and 5 new iridium spark plugs went in as well to minimize the ignition issues in the future. Boost pressure sensor decided to die so got a new one of that too.
Replaced ALL vacuum hoses including non return valves too. Had to replace the short rubber adapter between the turbo and the charge pipe that comes across the top of my engine. (it was leaking). This weekend I removed the oil pan and gave it an overhaul (new seals and unclogging the passages + new filter). Also I was able to look up in the cylinders and just do a visual check, wiggling the connecting rods to see if there is any play in the bearings inserts etc. Going to do a compression test in the coming days too. All of this paints a good picture how is the engine doing before tuning and I learnt so much too that I had no idea before.

New fuel filter and injector seals.
Fitted new slave cylinder, clutch plate, pressure plate and dual mass flywheel too. I had to admit it was hard to do under a pop up gazebo during the Christmas holidays but I got it done.

Front shock absorbers, top strut mounts, lower control arms, sway bar and drop links got replaced. I added subframe bushing inserts to the front too. General brake maintenance.
So many of these things were on there way out. Also fitted a boost gauge!!
I know its a lot but this it over a course of about 5 month.
What I am trying to say with this is I decided to keep and save this car for longer and have fun with it too.
I would recommend to go Stage 0 first and then go for mods/ performance upgrades.