The 24k service probably includes the (often missed) valve adjust which can take 10 hours and is a bugger of a job as it involves partly removing the engine. Because of this, many garages avoid doing this. The trouble is at this age, you cannot properly service a car (and make money) based on a fixed price menu that was designed around a 3 year old car. After 15 years in the UK things start to deteriorate, even in a low mileage car...which neatly brings me to your exhaust system.
The air injection pipes are part of the exhaust emissions system that was developed by Porsche to get around US smog. Basically the system consists of an air compressor which injects air into the exhaust system via 6 pipes which feed air into each exhaust port. These air injection pipes are mild steel and over time rust. A lot of people, myself included, remove the whole system when their engine is out as it weighs quite a lot, is expensive to fix and and isn't necessary to pass our MOT. The holes in each head can be easily plugged - most specialists will know how to do this (essentially all you do is just screw in a "plug"). If you aren't taking the engine out, you can just disconnect the compressor drive belt and plug the holes anyway.
If you are using an OPC they will be reluctant to do this as they will want the car to remain factory standard, hence them saying to replace these pipes and charge you accordingly...
Heat exchangers - although they do only provide heating, once they get holes in them the heating system starts to suck in air from around the engine bay - which doesn't smell too nice. So, I would do what I did when my heat exchangers were shot and buy some B&B headers. They will give you a useful increase in both torque and power as well as reducing spool up time on your turbo. I spent a lot of time reading up on the various options - Ninemeister, Cargraphic, FVD, GHL (no longer available), goingsuperfast and Hayward and Scott. Ninemeister were a B&B copy and very expensive (and I personally wouldn't touch them with a barge pole), Carpgarphics were horrendously expensive, FVD were just B&Bs with a large mark-up, GHLs didn't provide as much power, goingsuperfast were designed for the 930 and need quite a lot of modification and H&S were unproven and expensive.
Again, an OPC will only fit original heat exchangers - I was quoted £600 each 2 years ago by OPC Swindon. Fitting will be expensive as no doubt at least 1 stud will need drilling out.
The air con belt is easy to sort and I doubt will cost much to replace even if the OPC do it for you.
Good luck.
