I did N-scale trains for a while, and I can tell you that most engines made by Bachmann are mostly in the toy catagory. They have thick details and even thicker paint and they may run out of the box, but not for very long. You pretty much get what you pay for. A good running engine are made by companies such as Atlas, Life-like or Kato but will cost more. It is worth the extra price if you want something to work. Another bad choice will be Model Power which is worse than the Bachmann. Bachmann does market a Spectrum line wich is much better that their toy line, but still not as good as a Kato or Atlas.
To save a bit of money, you could consider used. The company I list below deals exclusivly N scale products by mail only, but buys collections and has a large assortment of used equipment.Scott, the owner, I have met and tests all the pre-owned equipment he sells and will state up front as to its condition and how it runs.
http://nscalesupply.com
Also if any train shows tour your city, bargains can be had there as well....
Z-scale I don't recomend due to the cost of the equipment. An engine in Z-scale can run several hundred dollars due to the size of the mechanism and the availability of the equipment. N-scale is much more popular and costs less due to mass production.