looking to purchase a powertap

i'm out of cycling shape and would like to get in shape as efficiently as possible. thus i'm looking into getting a powertap for my bike. i have a campy setup.

my options are:

- a used powertap on ebay (currently going for about 450)
- new from competitive cyclist/excel sports for 659
- a used wireless powertap on ebay
- a new powertap on ebay

what are the advantages of the wireless vs. wired?
is the wired for accurate?
what about used vs. new?
do powertaps have a long lifespan?
should i just save my money and get a used wired one on ebay?

and how about my timex HRM strap will that work with it?

thanks.