You want a ssd in whatever system you buy. An old machine (even 2-4 generations old) with a fast ssd will run rings around a "new" machine (i5-7400 is one generation old at this point) with a POS 5400 rpm spinning rust (hdd).
If you need a ton of space, get a reasonable ssd (128 min, 256 may be nicer) and some spinning rust for your mass data storage.
I'm a build it myself type for desktops. It isn't rocket science, you could likely DIY if you wanted to. You are on the mustache forums after all.
That said with prices being (about to?) be inflated by the trade war, might be worth looking used on craigslist + ram/ssd upgrades -- just be sure to let windows to a refresh/reinstall. You don't want any 'gifts' left by the seller on the machine.