Lots of different options floating around here, probably worth thinking about what sort of form factor you'd want - desktop tower vs all in one, also worth considering if a laptop may be a more suitable option. For refurbs (or any machine to be honest), I'd always go down the route of the business oriented machines such as the Dell Optiplex line over the more consumer focused options such as Inspirons - I find they tend to be better made and often easier to get replacement parts for.
I'd definitely second Lisa's advice about an SSD, easily the biggest performance boost you'll see for day to day use. For general office type tasks, a lower end CPU paired with an SSD will feel hugely more responsive than a top of the line i7 machine running from a conventional hard drive. Nowadays I'd never buy a machine without an SSD boot drive.