The main difference in old machines and newer models it the coming of the digital age. The newer ones might have a bit more accurate discriminatin but most really good hunters work a lot in the all metal mode anyway. Some of the "modern" machines have been around for several years and not much has changed, IE: XLT, GTI2500, etc. I'd look for one that you can still get different size coils for or that already have them and one that is still currently being made or has just recently ben discontinued. Also, you will need a manual and most dealers have them available, some even on line. monty
PS. I am predicting some really inovative equipment coming on the market really soon and you could always trade it back for a newer model if you buy from a reputable dealer.