There's nothing wrong with refurbished PC's as long as you get them from a reputable supplier and they come with a decent warranty, and Windows 10.
As long as the PC has at least 4Gb of RAM and a processor that's reasonably quick then BetAngel will run on it without any problems.
I'd recommend a Windows 10 based system as it would have to be reasonably newish to be packaged with the operating system. It's not advisable to buy a PC without an operating system unless you know where to get OEM versions and have no problems creating bootable CD's or flash drives.
If you want to run more than one BetAngel session at once then you'll need more processing power. I have a dual core system for automation testing that's a bit old but it still runs two simultaneous BetAngel sessions well enough, even on fast streaming refresh rates, although it does take up 60% to 80% of the resources of the PC and can't run anything else at the same time, or else it chugs like it's steam powered. I have a fast laptop for the real heavy lifting.
A lot of standard graphics cards have dual outputs, and they're very inexpensive, and some PC's come with them as standard. If you want to go for 3 or more screens you'd have to invest in a more expensive graphics card, but even a decent GPU is quite inexpensive these days.
Something like this will run 2 monitors without any problems: -
http://www.ebuyer.com/737926-msi-geforc ... 0-1gd3h-lp
One important thing to add is that dual output graphics cards usually have more than one type of connector (e.g. HDMI, DVI or VGA). It may be wise to make sure the monitors you use have multiple connector options. Additional cables on ebay are cheap enough, and you can get converters for HDMI & DVI if needed.
If you want to run more than one version of BetAngel at the same time and use Excel and t'interweb simultaneously then you'll need a decent processor with as many cores as you can get for your money.
I'd recommend a decent Intel i3 (AMD A6) processor as a basic starting point, an i5 (AMD A8 or A10) system would be my personal minimum if I were purchasing a new PC, and an i7 processor would be highly recommended, if budget allows.
Core 2 Duo systems and equivalents just can't cut it I'm afraid, so stay clear of these, and pentium and celeron processors too. They're just too slow.
If you need any further advice or assistance just PM me and I'll help as much as I can.
Hope this is useful.
Martin