I use a old 2009 windows 7 for mine---- works great
Check out KC0CF's web site for great used laptops ready for SDR. He post on QTH under Computers and accs
I have bought a couple of the older cheap ones and they work great!
Please go for a machine with excellent processing power. Ideally I7. The second most important aspect to consider is the compatibility of the Firewire port to operate Flex3K. I have had good luck with Dell and Sony that used to come built in with the Firewire port. The modern ones do not. So you will need a compatible firewire adapter. If you are in the USA visit <http://www.abrohamnealsoftware.com/> Neil used to sell a lot of Flex compatible hardware and Flex ready machines
The key to a good performing laptop is low DPC latency. I have seen Bluetooth and WiFi adapters that generated long duration DPCs to the extent that the PC was unusable with a FLEX-5000/3000. And cost is not an indicator of good performance. I purchased a high-end Dell portable "workstation" that was unusable.
I know people have had good luck with some older model ThinkPads.