Hi Tigger,
Low oil pressure (caused by blocked pickup) can cause noisy chain, or excessive wear on chain. But it can also just be the chain & tensioner have generally worn over time, causing it to rattle. As I understand it, the chain tensioner is driven by oil pressure, thus the oil pickup and chain tensioner are loosely related.
Prices as follows, I've had the chain/tensioner done and plan to have my oil pick up inspected some point soon, work for this has been quoted by my local specialist.
Timing chain/tensioner:
Parts = £186.10, all genuine parts as listed here:
http://www.mk5golfgti.co.uk/forum/index.php/topic,109900.msg1063394.html#msg1063394Labour = 4-5 hours. (cost me £360)
Oil pickup pipe:
Part (oil pickuup, maybe not needed as old one can be cleaned) = £43.
Labour = 1-2 hours, inc oil change. (factor in new oil costs too).
Personally I had a rattly timing chain and just the chain and tensioner replacement sorted it. But for piece of mind at next service ill be having my sump dropped and oil pickup cleaned or replaced too.
As for the rust issue, its a common one. a simple google of "mk5 wing rust" will return plenty of results and there is a post on here detailing other owners' experiences with this.
http://www.mk5golfgti.co.uk/forum/index.php/topic,48873.0.html 12 years is the corrosion warranty, this post shows mixed results in temrs of gettig VW to pay out!
Hope that helps!