Hi Newby82. Buying second hand is OK if you meet the seller, check the serial number, check with dealer who sold it, get a receipt off the seller from the dealer. Take someone who owns a 4500 with you to test it out. Do not buy anything that's been modified.
The best advice I can give is to buy a detector that suits how much time and effort you want to dedicate to the hobby.
If you plan to put in a at least a few days every month detecting for gold over a couple years or more and you are prepared to go out with some experienced people and read the manual 3 times, and really test different settings and write them down, and if you plan to find more then a few crumbs of gold get a 4500 with signal enhancer/speaker, with the standard coil, a 12 inch round mono coiltek or Nuggetfinder coil and a small coil 8inch. You will need to become a student of the detector and become a little obsessed with getting good at the machine.
If you plan to go out occasionally, mainly on old digging and find a little gold.... get a 2300
IMHO do not consider getting xterra or similar...you will waste all of your time and money and walk over gold.
Cheers RDD