Besides the various 3xxx and 4xxx series machines
mentioned a
 MicroVax2000
 is also a possible machine and by far one of the smallest.  It will run
 V5.4 and
 if you tailor it V7.2 (disk space is limited to a MFM rd54 of some
 159mb).
 Versions of VMS out there worth considering V5.4, 6.2 and 7.2 are common. 
I'm going to have to disagree with Allison on this one.  I strongly believe
that a VAXstation/MicroVAX 2000 is not a good system in this case.  There
are two major problems that I see, both related to the disk interface.
You're stuck with a MFM disk, all of which are getting very old, and you
can't connect a CD-ROM.  If you're only going to have one OpenVMS system, do
yourself a favor and get one with real SCSI!
Also, why V5.4?  The preferred landing zone for V5.x is V5.5-2.
                        Zane