If it's a k03, those injectors dont fail as bad as the k04 ones do so you're safe for te time being. also they are way cheaper than the k04 ones even brand new oem.
DSG service can be a hit or miss. If not done right, it'll run worse afterwards. Whoever does it needs to know exactly what they are doing.
Suspension is also relatively cheap. Throw in some Bilstein B4 sport version shocks and it's better than new. You can keep the gti springs or get black Eibach ones for a very slightly stiffer suspension. There's also the front suspension bushing which you can replace with the S3 ones also for super cheap.
Timing chain. You only really need to touch it if it rattles, it gives errors or your throttle hesitates/hangs. This one I suggest going all out and replacing everything relating to it. Chain, tensioner, adjuster sprocket, controller.
Windscreen is definitely worth it if worn. Night and day difference. Dont worry about the rust around it, just tell them to apply some windscreen glue there and to lift the glass higher. You wont even notice it and the glue keeps the water away from the metal so it wont rust further.
If you do disks, do pads as well. This is costly but it will freshen up your braking feel if done right.
Whether you should keep it or sell it is entirely your decision. do you like the car and dont mind investing in it or would you rather hand it off and get something newer which is much easier to make power on since just a stage 1 remap will make more than a k04 mk5 will. It's also more comfy, quieter and overall looks better (imo). I gont like hatchbacks that much but if I were to ever get one, it'd be a mk5 cause they really nailed the shape and it's sharps
My honest opinion? If you dont have any special attachment to your mk5 nor have you really invested into it and you have the option to go for the mk7, go for it.
