For you budget and type of riding, I would consider a full suspension with rear lockouts. This way when you are using the bike for commuting you can lock out the rear suspension, and don't lose too much energy from bobbing up and down.
Just a few suggestions when locking up your bike, do not use the same spot more than a few times. If people see it in the same spot the same few hours, it makes it a target.
Take it into a building as much as you can, if a store doesn't let you, then don't go in. It takes seconds for someone with bolt cutters to get your bike, even thos "U-locks" are easily to break into.
My suggestions for a bike, are my personal preferences;
-Trek Fuel EX
-RMB Element, or Vertex (Hardtail)
This time of year, I would not recomment to buy from a store, the prices are at a premium. Buying used is another storey, avid riders are upgrading right now, and selling off last years cheap.
Ebay is a good choice too.
http://cgi.ebay.com/TREK-Fuel-70-Ful...item2eadbb26e4
for example
Unfortunately I have a 16.5" Fuel 80 that has gotten maybe in 100KM in the last 3 years.
Ken