I would get a new bike, if only for the warranty. All the bikes in your list are probably very close in spec and in quality, so go for the one that fits best.

I also wouldn't worry about getting the 'perfect' bike now. Get something that will get you out and riding, and if you stick with it, by the time you outgrow that bike you'll have a much better idea of what you want in a more substantial bike.

My example: I basically over-spent a bit on my first mountain bike - got a $1k aluminum hard tail when a $500 steel one would have been fine. I rode it for a good year and a half. By the end I had figured out what kind of terrain I liked, what types of riding partners I preferred, etc. and then invested in a kick-ass bike that will stay with me for a long, long time.

If you get in even moderately better shape, you'll probably have a much better idea of what kind of stance you want (upright vs. forward), what features on the bike (fat tires? fenders? braze-ons?), etc. Then, you'll probably get a bigger budget from the boss.