Make sure you test run it before you buy it, and check all of the wheels with an NMRA gauge. Many of the early ones have wheels that are narrow in gauge, causing derailments on turnouts. My LHS still has about 5-6 of the first run units on their shelf!
The drivers cannot be simply re-gauged or they will crash into the valve gear. The trailing truck is a metal casting which must be relieved on the inside before its wheel gauge can be corrected.
Also, power pickup can be a problem - many had the drawbar installed incorrectly, and the pin on the locomotive chassis usually needs to be cleaned up. Plus, mine had the white tire paint slopped all over the tread surface of the drivers.
I would never buy this or any Bachmann locomotive without inspecting it first!