Height adjustable desk is nice-to-have rather than essential (would hate to go without it though, since it allows me to stand while working once in a while).

But external monitor should rightfully be mandated by law, IMO. Looking down, even slightly, puts a lot of strain on your neck muscles and can easily cause injury over the longer term. Your head and neck is like a bowling ball balancing on a pencil - fine when it it right on top, but dangerously heavy if tilted slightly off center.