At that price range they are all about the same, and come with the same amount of bloatware. Main things to keep in mind is they cut corners on things like the power supply, which is the most likely thing to go out; and if you can get your hands on install media to just do a fresh install out of the box you get rid of the bloatware easily(I hate bloatware, and it must die painfully). As far as hardware reliability, ASUS is a brand has done pretty well, but I don't know what you can find in that price range for desktops. ASUS tends to focus on mobile more.
8.1 is fine, if you can stomach avoiding the Start screen. There are programs to give you the start menu back if it is important to you. Depending on what you do, it has a few quirks, but basically works like Windows. Pin the things you use to the taskbar or make icons on your desktop, set it to boot to desktop directly (next major update coming out in a month or 2 here is supposed to enable that on desktops by default), and use it just as you always have. If you use wireless networks, know you can't remove them without going to command line. Other than that, you should be okay with regular internet browsing use.