The problem with buying used is that it is very difficult to evaluate a used bike without a fair amount of experience. Even if you are very shrewd, you still don't know how the bike's history and you are always taking a chance.

Stop into a local bike shop or three and see what they have available in your price range. The fact of the matter is that the brand really does not matter that much, especially at your price point. The bikes will be very similar in quality, so you should make your decision mainly on how the bike feels to you. I'd look for a rigid frame and fork if one is available or, failing that, a hardtail (rigid frame and front shock). Remember, if money is going into a suspension then it is coming off of somewhere else, which is why I would very strongly recommend against a full suspension.

Now, having one bike to ride on- and off-road is going to be a small compromise. Knobby tires really slow you down on the road. If the trails you are exploring are hardpack, then I would recommend putting on something like a 2.0" slick or semi-slick, balancing your choice of tire profile against how you split your riding time. MTB gearing will be too low to race on the road but it should be enough for most riding and commuting. If you think you will be on the road a lot more than off then you might also want to consider a hybrid.

You should be evaluating the shops as well, since you will want to have a place to go to if you have a problem and when you need an adjustment made, which you inevitably will. Most shops will give a free adjustment after about 30 days to fine tune things as they have 'bedded in'. They should be making sure that the bike fits you and not simply pushing the remaining sizes they have left over from the summer. Also, they should be willing to make minor adjustments to the riding position to help 'dial it in'. You should be able to get a good price at this time of year so don't feel a need to rush into a sale.

I'd recommend staying away from department stores and especially the full suspension bikes which they carry. Besides using inferior components [the 'Shimano' label notwithstanding], they typically are not always very well assembled and usually require adjustments to even be considered safe, let alone comfortable. Beyond selling you the bike or accepting it's return, department stores are of no help to new riders and this is the difference a good bike shop makes which you should not underestimate as a newer rider.

Do some reading at http://www.sheldonbrown.com and consult the glossary ( http://www.sheldonbrown.com/glossary.html ) for any terms relating to bikes which require explanation. Also, don't hesitate to come back here with more questions before you settle on something.