A nice Macbook air would do the job IMHO if you want to be able to work on the go. I used to work on an air for years, using Sketch without any serious issues.
If you are thinking on a desktop computer, I would recommend to buy the parts separately and build it yourself or ask a friend to help you. Also, another alternative you can look at is https://hackintosh.com/#hackintosh_compatible
A nice Macbook air would do the job IMHO if you want to be able to work on the go. I used to work on an air for years, using Sketch without any serious issues.
If you are thinking on a desktop computer, I would recommend to buy the parts separately and build it yourself or ask a friend to help you. Also, another alternative you can look at is https://hackintosh.com/#hackintosh_compatible