For the brand I'd choose a brand with next-day on location hardware support. I've seen Dell offering this in the past. It's much nicer having a tech come to your office the next day than having to send it to them and wait 3 weeks till it returns.
For the OS it's up to you. The work you do and companies you work for might be important. If all you need is a browser, Linux is just as good (some say: better) as Windows or MacOS.
For graphic design, some prefer a Mac.
And if it's needed you work with a Sharepoint database or have to run software that only runs on Windows or have to share complex spreadsheets it's Windows 10 and a recent version of MS Office. Or Windows 7 if you need to run custom software that isn't compatible with Windows 10.
I badly need help! Please recommend OS and computer brand.