It's not a bad gateway to building your own computer eventually. Before I built my first from scratch, I upgrades an off the shelf emachines desktop a little here and little there. The more you learn that way, the less you have left to learn when you build from scratch. And if you're like me, once you do build from scratch, you're going to want to buy parts that will allow you intermediate upgrades before your next full build so you can drag out the life of your PSU, case, optical drive, possibly an aftermarket cooler, ssd and only replace a few items. Nothing wrong with starting at that intermediate stage.
If you buy build from dell, might be a medium case, you have to be extra careful about the size of everything, cpu cooler, RAM height, GPU thickness. My first case, and the one I still use, is a full atx. I never have to comb through reviews looking for caveats on components, I can just shop the sales and get what I want quicker and cheaper.
As to gaming on medium settings, well, that's most of my experience. The only time I don't game on medium is when I'm gaming on low :wink: Not knowing what high settings look like probably makes it easier to do without.