If you intend to keep this permanently fixed in one place and aren't trying to do proper museum-level historic restoration, I advise that you buy one which is totally not roadworthy, where the mechanicals are trashed. It'll be cheaper for you and it'll leave the ones which can actually run on the rails for the museums and restorers.
For a break room, unless you're planning to cook full meals, you probably want a lounge car or cafe car or "table car"; you probably don't want half the space to be taken up with a full kitchen. If your boss is willing to pay for some renovation work, a suitable coach car -- one with all or most of the seats missing, which is actually pretty common -- is pretty easy to convert into a table car too, so that gives you a lot of options.
I'll warn you, practically all used railcars seem to need window repairs so budget for that. Since you're not going to run them on the rails, it'll be cheaper than if you had to use proper "railroad glazing".
There were a lot of restaurants which used grounded railroad cars and trolley cars as decorative items indoors (people could eat in them), particularly Spaghetti Warehouse. They removed all the seats, restored everything cosmetically, and have an open floor suitable for putting whatever furniture you like in. If you can find one of these which is no longer in use by the restaurant, this may be the minimum amount of restoration work.