The cost of bike repairs is mostly labour. If you can do the work yourself, the parts will likely cost less than buying a used bike that's ready to ride. I ride a 1996 trek mountain bike in winter, and it's still holding up nicely. I replaced the drive train a couple summers ago, added fenders and rack, and it's still running smoothly now. New brakes needed every couple of years, and I'm thinking it might be time for a new wheelset. If you don't think you can do the work yourself, then you're probably better off finding a bike that's ready to ride. You should still get comfortable enough cleaning and lubing your chain, repairing a tire, the kind of basic stuff that paying a mechanic to do would be too inconvenient, or too expensive.

For a first bike (in a while), I'd agree with the above and go used. I ended up buying my wife a lower-grade new bike a couple years ago because it was very hard to find anything small enough used, and now it's looking like it's probably time to replace it with a better model, so that one isn't going to see a lot of years of use. A mountain bike can make a perfectly functional commuter, and it's better to find out what you like and don't like without spending hundreds of dollars.

I've never had a chainguard, but will certainly admit to getting my pants dirty. On some level, it'd probably be a nice feature, but would make some maintenance activities more challenging.