IF you buy new, have the LBS retension the spokes as part of the deal.
That can make a world of difference in the life of the rear wheel.
Try to get 36 spokes vs less.

If you look at your rear wheel, imagine what happens when you pedal. 1/2 the spokes are trying to "unwind" and thus go relatively "slack", while the other 1/2 get "stretched". This tends to flex the bent part. IF the tension is uneven, some bend more than others and metal fatigue will shorten their life drastically. IF the tension is equal, they better "share" the load and cumulatively, bend less.

IF you listed your location in your info, we could help look at the CL ads for you.
A $500 budget is pushing it for a new bike that will suit your needs.
Spend 1/2 that on a used bike and you have some "reserve" money to set the bike up to suit you.

Look for a bike with a Free Hub, instead of a Free Wheel. The latter will tend to bend axles with little effort.
http://sheldonbrown.com/free-k7.html