Coldfeet mentioned this already, but thought I would point it out as well. You're buying a used bike. Chances are, you'll need to replace things, if not right away, then in the not too distant future. Brake pads are the most frequent, the chain needs to be replaced occasionally, the sprokets may be worn and need to be replaced. The tire may be old and worn, or may be the wrong type for your needs. You may have to replace the saddle if this one doesn't work for you, and if you're on the bike 4 hrs a day you definitely want a good saddle. You may have to get new peddles. Other things you may need to buy, in addition to what was already mentioned: a multi tool kit, a rack, a seat bag or trunk or panniers.

This is why I would not suggest stretching the budget on the bike itself. There WILL be other charges down the line that you need to be prepared for. I would look into a new bike well within your budget (new bikes have a warranty and usually will come with free tuneups) which will let you buy all the things you need without running out of money.

Alternatively, if you really want this bike, try to find some way to increase your budget.