I'm assuming with a 20-mile commute, 3-4 times a week, and carrying nothing you'll be showering and changing once you get to work?

If so, you're going to eventually want to ride through rain and maybe even snow, depending on where you live. So you'll need fenders - but no, you don't need fender or rack mounts, though those do make things easier. Google "p clips". They work. Marine-grade stainless ones will probably outlast your bike. Though you probably don't need fenders right away.

The best tires for your situation depend on what you're commuting on. With no loads to carry, riding only on roads/paved trails means 25s are more than sufficient and are probably the best choice.

I'll opine that you want at least Tiagra level or 10-speed shifters. Don't worry too much about the other components like derailleurs or brakes - bikes with Tiagra or better shifters will almost certainly have other components that are good and reliable. 105 is nice but is pushing your budget.

And regarding that budget? You are going to wind up spending a few hundred on other stuff like cycling clothing because you won't want to wash the same two pair of shorts twice a week. And if you do want to do that, you'll just have to replace them faster. You might find out the hard way that riding that much means you need to get better/more expensive shoes or shorts. You might want to get another helmet or two because they need to air out more than 24 hours between rides.