zambusa wrote:
Thanks for all these replies - much appreciated! It's not an easy decision so I will probably sit on it a while longer and dwell on the above comments

. This would be pretty much a shoestring project - pricewise I'd be looking at most at a 400mhz Octane [or 360 dual], or maybe a 500mhz Fuel, or a low-end r12000 o2. Maybe even an Indigo2.
Probably at first I'd want to plug it into my kvm so I'd want something that takes ps/2 keyboard at least - needing to get a mouse doesn't matter so much, I've ended up with 3 in use for 4 pc's so another one won't make a lot of difference

.
However - regarding sgi ps/2 keyboards:
- are these likely to work with a kvm for both the sgi and pc's?
- what's the action like? are they rubber-dome type? I'm guessing they aren't buckle-spring but I saw a reference to ALPS relating to sgi kb's somewhere, wasn't sure what that meant.
On the mice I'm assuming I would need a proper 3-button mouse - a windows wheel-mouse won't do? Or would the middle button work, though not as a wheel?
Monitors...
Mine is a Lacie electron 22 blue iv aperture grill crt - I imagine that it's something else rebadged [sony, mitsubishi, iiyama, etc], but it does 1600x1200 [I think up to 2000 or so horizontal in fact]. It has 2 vga inputs, I guess I will google to see if it can work with a vga/sync-on-green cable which I guess I will/may need for Octane or Indigo2.
I think you mentioned you were looking in the UK, and some of your comments echo my starting point, so let me share my experience:
I had a KVM with an unusual PC (running Debian) and a RiscPC, so also looking for a PS/2 machine that would be a foray into workstation territory. SGI because I had seen it in action as a molecular modelling tool in the mid-90s. Limited outlay for a hobby/experiment: 40 quid on an Indigo2 last March, plus a bit for postage and a 13w3-VGA converter. At the time there was a cheap O2 (though only a R5K) and an I2 Impact, just after I'd bought (so there is never a best time!).
Result: knockout! Nekochan support/enthusiasm really good, IRIX and the IRIS desktop seemed great, since bought a Fuel. Yes, does work on a KVM, but not really at the same time as the other machines; does drive VGA nicely (both I2 and Fuel have been fine with my 17" Iiyama CRT). Scroll mouse support there from IRIX 6.5.16, so get the free upgrade to 6.5.22 from SGI. This works through the KVM (despite comments to the contrary) although not with all applications.
For a Fuel, on ebay "usedunix" did have some on "Buy it now or best offer" so worth seeing whether your budget counts as an acceptable offer. There has been a 500MHz at around 200 quid lately, as well as a fairly low-spec O2. Decent O2s are priced higher, and there don't seem to be many Octanes around. Unless there is an application for which you need dual-processor, I would recommend the Fuel. That said, once you have one, the Tezro becomes an "I want one of those" item!
Whereabouts in the UK are you? there may be someone willing to lend you a machine for a taster, but better to avoid postage if you can - SGIs are heavy!
Also worth a look in the nekoware and SGI freeware catalogues to get an idea of available software (and recent-ness of versions) compared with what you want to do. Some things are kept reasonably up to date, but no OpenOffice beyond version 1 as far as I can see!