Don't know about the 8620 specifically, but I've got a WavePro 7200 and by the look of the specification, a lot of parts are probably similar to the 7000 line. The interface should be as you expect, since the X-Stream software goes back for a while, but the datasheet I saw mentions Windows 2000, which would mean you're limited to upgrading the software to version 6.8 or something - if you install XP on it, you then get access to more recent revisions of the software, which I believe do officially support this scope (maybe not the newest versions, but version 8.something, at least).
You probably already know if you're looking into this bandwidth, but you're going to need ProLink adapters just to get an SMA connection on an input channel, and those high bandwidth probes do not come cheap and if you buy used, often don't come with accessories. Also worth mentioning the frontend is only good to +-4V, so treat it gingerly!
From the spec, looks like the edge trigger is only available to 5GHz, not the full 6GHz of the unit, and the fancy SmartTrigger options are only available to 750MHz.
Otherwise, the layout is probably similar to a lot of their scopes from the mid 2000s, so you'll probably have an off the shelf mATX board as your computer and a regular hard drive mounted to the back chassis - so upgrading to get a bit more compute performance or lower loading times is probably easy.