Matt & Josh -
1) Occasionally I see them offered on eBay or CraigsList (used), but because AMB has an airtight lock on the market and you have so limited a choice on where to buy, the used price usually isn't that much of a discount.
2) The 160 will work -- if you mount it low enough and the unit is completely clear of anything that will interfere with the signal. Remember, these were designed to be mounted on the frame-rail of karts (2" off the ground). If you're running a Lemons car now where you can mount the 160 low and protected, do it and save the $100 price difference... HOWEVER... please consider that the transponder will probably far out-live your Lemons car (I've had my rechargeable 260 unit for 7 years) and, as such, unless you can guarantee that your next Lemons car, and your next (etc.) will all permit similar mounting (low, unobstructed and protected), you should probably just bite the bullet and get the 260. The hardwire is also probably the better choice for Lemons, as Lemons cars all have alternators (at least they do when we start the race). I originally used mine in Formula Atlantic and so I bought the rechargeable, battery operated unit. Still works great but I suspect one of these days/years the battery will stop accepting a charge. Now that I'm in a closed-wheel car (with alternator), I note that most people in my class have the hard-wire version.
John
"Age only matters if you're a cheese." Helen Hayes