You also wouldn't have to buy Windows computers - you could either buy Windows + Parallels or spend about $60 a month and get a hosted Windows desktop on AWS.
You can get a hosted build server via Visual Studio Online - unlimited private git repos, 5 users free (each additional user $5). 240 build minutes free per month or $40 per hosted build agent with unlimited minutes.
If you really have no need for Windows machines, just use AWS Workspaces and you can get a 4 vcpu 16GB Hosted Windows workstation for $70 a month.