The hardtail idea is smart, get an HQ bike rather than an entry FS.
Commuting, maybe you can do a tire alteration, I run a semi-slick rear to reduce the rolling resistance.
Are there lbs near you? =you need to ride a few bikes and see what works for you.
You also need to figure what size of bike you ride framewise, and decide what type of HT you would like.
Types are XC, All mountain, Freeride. The best for all around might be into XC, the bikes have less travel forks, are ridden with the seat higher and pedaled while sitting.
Most downhill, freeride bikes are run with the seat low, they have long travel forks and you can jump them.
Respected names such as Kona, Trek, Specialized, Brodie all have good bikes, I'd stay with a company that first was a mtb builder.
Marzocchi is the premier suspension fork company.
SRAM is a good drive component company, Truvative also.
Shimano gets a bad rap, but the higher end stuff will do.
You need to run a few frames to see what works for you. Shoot a little lower than your total $$$, to buy a helmet, padded gloves and a bike tool.
Stems can be purchased to get the 'perfect' fit if you don't get it done @ purchase.
Buy from an lbs for the free tuneups, and often they will swap out a component or 2 within reason to get you set. A good lbs relationship is useful, and since you may need some repairs -the place you bought the bike is the best place to go.
The $500 mtb thread stickies should give you some general ideas.
http://www.bikeforums.net/showthread.php?t=56148
http://www.mtbr.com/ has reviews on bikes\products.
Find some bikes, and ask some opinions.
I have no idea WHAT bike you should buy.
Maybe a chromoly one?