Browser version is always the current for whatever the browser is labeled (Chrome stable is 64 for example) - at least within a day or so. IE is the only one that has locked versions and those are in the description. If you want to be really sure you can look at the UA string in any of the requests and it will have the browser version info. The version is also exposed directly if you are consuming results over the JSON API (just not displayed in the UI).
The OS is also in the UA string but will vary by location. The actual laptops (Thinkpad and aspire) in Dulles all run Windows 10 (latest update). Most of the other locations have been migrated over to Linux.
As far as hardware goes, the only physical machines are the laptops in Dulles. Everything else is a VM and the capabilities will vary. For consistent cross-location testing I recommend using the EC2 agents which are all the same c4.large instance size.
Anything that requires a physical GPU and a relatively high-end machine should run on the Dulles Laptops.
If you run your own private instance then it can run on whatever hardware you decide to use for testing (if you need a specific configuration that isn’t available on the public instance it’s the route I’d recommend).