REFURBISHED DELL PC OPTIPLEX 360 CORE2 DUO, 2GB DDR2 160 GB HDD DVD RW, CAN DELIVER, LOTS EXPORT | United Kingdom | Gumtree
would do for start up and should get the 3 of these with monitors off him for £200 collected
get rid of vista and install the basics you need
those are a little slow but should be OK. £80 a piece inc monitor. Add on £10 per PC for keyboard and mouse and possibly power leads. I assume you are not fussed about legit licenses for now ?
As mentioned, swap the hard drives for 120GB Samsung 840 Evo SSD for £55 or £90 for 250GB version. These can always be re-used in the future.
Couple of other items, somebody has already listed a colour laser, I've got the non-ethernet version and its a very good printer. Input tray is a little short on capacity (I think 30 pages or so) but its compact, quiet, quick and good quality prints. You can get a set of compatible toner for around £60.
Final two things concern managing data. First get a USB hard drive for backups, £50 for a 1TB, will keep you going for a while.
Second, consider getting a 4th PC plus 2TB WD RED drive to use as a NAS device. This will centralise the data and make it easier to share, manage and backup. You can then get away with smaller disks in the PC.
Is the office cabled for networking ? If not then you have the option of wireless or if possible better to cable it. You'll probably need an extra switch, that would be around £20.