Greg, whether you decide to purchse this car, or a different Boxster, make sure you get a Pre Purchase Inspection (PPI) done by a qualified Porsche shop. It can be a dealership or an indy shop. Expect to pay a couple hundred dollars.
A little bit of tapping or ticking noise at start-up is normal. Especially from a cold start. Should only last a few seconds, until there is good oil flow and pressure.
It should not be "loud" (as you describe) and all the time. You need to get this checked-up and better understood. Could be as simple as low oil or even the wrong oil viscosity (should be Mobil 1 0W40). Could also be more major like a damaged or stuck lifter.
Try to find another 2005 Boxster, and listen to how it sounds for a comparision.
Once you get a better understanding, and know whether it is an issue or not, proceed with caution. If everything checks-out OK, and the noise still bothers you, get a louder exhaust!
