32 bit Sun Sparcstations need proprietary keyboards/mice and adapters to work on normal monitors, but they at least used to be about the cheapest entry level UNIX workstation and the easiest to find. (The Sparcstation IPX, IPC, 4, 5, 10, and 20 were all sold in relatively huge numbers compared to most workstations.) They also run a fairly broad assortment of alternative OSes, the exact choices depending on the model or hardware config.
Of course, this is me remembering 20 years ago when it was pretty easy to find someone willing to pay you to haul one off, no idea what the market is now. I also think a lot of them still around will need TLC to get going. (I recall several of the most common models used those annoying Dallas clock chips with the built-in battery, those will all be dead by now.)
I'm really late replying (and haven't looked through the whole thread), but that being said - IMHO, Sun 4c/4m is probably the easiest UNIX workstation to get into. Sure, the keyboards are proprietary, but you should have NO trouble getting a Sun 4 or 5C keyboard and mouse. I personally prefer the 5C, though I never liked how the keys felt (I'm an IBM Model M snob). Don't forget about the Sun 1, 1+ and 2. I think those were sold in relatively high numbers. My firsts were an IPC and a 2. Don't have those I originally had, but do have a 1, 1+, several 2's, an Ultra 2 that I still use to service other machines, and I collect Sun 2's and 3's.
SPARCs are pretty easy though. Like I said, the keyboards are plentiful. All you really need to connect a modern LCD to a Sun color framebuffer is a 13W3 to VGA adapter and a monitor that supports sync-on-green. I'm very partial to NEC Multisyncs (not all support SOG but it's easy to find out if they do by looking up the model number and looking at the specs from NEC). I also understand a lot of Dell LCDs support SOG.
I would say that the Sun4c (SS1, 1+, 2, IPC, IPX) would be the easiest to get started on. I think the IPX uses normal 72 pin parity SIMMS, and I know you can use regular 9 chip parity SIMMS with the 1, 1+, 2, and IPC, either 1MB or 4MB capacities. All by default output console to serial port A at 9600, 8, N, 1, and IDPROM batteries can be hacked to solder replacement batteries/battery holders, or I think you can get a replacement battery. I know Sun 4m systems like the 10 and 20 are a bit more difficult. Proprietary memory, sometimes CPU modules don't get work, and don't get me started with how hot the SS20 will get. I have one with quad Ross HyperSPARCs and it gets HOT HOT HOT!