Keeping their software segregated is sound advice, but as they are your client there are a couple of other ways I'd offer to handle it:

1) Keep all software related to work for their company segregated inside a VM. Then you can install whatever they require without interfering with your main system or potentially exposing data for other clients.

2) If they want a separate physical system, tell them you would be happy to provide it for a fee: an upfront fee for the cost of the system and an ongoing fee for maintenance of it. Be sure to mark everything up as you don't work for free.

Since you're not an employee, you really shouldn't be asking them to provide hardware as (in the U.S., at least) this could create tax problems.