I love collecting old machines also <3 I have a PDP-8 and 11 replica (from obsolescence guaranteed), some of my first computers, a G3 iMac of which I just love the design, HP-UX boxes because they were really formative for me with Unix. I also owned some Sun hardware (a SPARCstation 5 and an X-Terminal) but sold it when I moved to a small apartment. I still regret this.
But indeed the servers are really hard for collectors because of the size. Similarly with the real PDP's of the day. We had a real PDP-11 at the computer museum where we volunteered though we never turned it on. Too afraid of a cap blowing and damaging more.
HP had some nice small servers later on, but they were not very influential so like you I don't really see a benefit in collecting them.
There is also the MicroVAX. We got one (a MicroVAX II) at my place of work shortly before I left in 1986. It ran Ultrix, a BSD derivative. As I recall it, the box would not have been too big for personal use, though the price tag probably was on the steep side.
From before, we had a VAX 11/750. It, too, was a freestanding box in the computer room, possibly close to a cubic meter in volume. Not something I would want at home! It ran VMS, which I found clunky and difficult to work with. But we got a Unix emulation layer called Eunice installed on it, which made it more bearable.
And yeah, the desktop VAXen were almost exclusively for VMS. But the standard Unix box in the early-mid 80's that everyone talked about when they discussed/benchmarked/whatever the platform was a 11/780 running 4.2BSD. Those aren't meaningfully collectible for an individual, obviously.