Run to the Appalachians to get away from any 32bit machine! Most here will advise to get a 4,1 and newer.
Here are links to cMP locations that include 1,1-5,1
http://www.usedmacs.us/?name=Mac-Pro&Scategory=1077648508
http://www.macofalltrades.com/Refurbished-Used-Apple-Mac-Pros-s/298.htm
http://www.dvwarehouse.com/Apple-Mac-Pro-c-5210.html
http://ibuildmacs.com/
https://www.powermax.com/productcategory/used-macs-mac-pro
If you post the desired use of the MP it will be easier for others to narrow down which model is doable.
I've actually been talking to someone in the Marketplace about a lower spec 4,1, and I like the better compatibility that 64 bit EFI brings.
I did do some reading this evening on installing newer OSs(specifically 10.9, as I don't like 10.10) on 32 bit EFI machines, and although it sounds doable it looks like I'd be looking at a GPU upgrade at a minimum. It seems interesting to me that folks are getting 10.9/10.10 to run(albeit through hacks) on Intel GMA950s in Macbooks, iMacs, and Minis, and by most standards the GMA950 is a steaming pile of crap.
In any case, I'd be looking at this as a replacement for my office computer-a dual core G5 2.0(late '05). My requirements generally aren't that strenuous(primarily Office and Chemdraw) although I do occasionally do some molecular modeling and also sometimes heavy Photoshop work on images(lens distortion correction, etc). I'm getting along fine with the G5 and 95% of the time it's not a bottleneck. I've recently purchased a G5 Quad, primarily for my Apple collection but also as a potential replacement for the dual core.
I'm looking at a Mac Pro because I do want a desktop and the expandability that comes with it(specifically I want to have a big internal Time Machine drive, an SSD for the OS and programs, and a big platter drive data storage).
I'm also finding myself a bit limited now by 10.5.8. I'm a PPC hobbiest and will continue to be, but I'm finding increasingly that programs necessary for work(specifically communicating within the University systems) are requiring 10.7 or newer, and a small subset of those need something even newer than that. Our department shared printer doesn't even have drivers for anything older than 10.8. So with anything I buy, current OS compatibility(as I said, 10.9 for now) is a must, although I'm going to have to bite the bullet and get used to 10.10 for Office 2016.
So, it sounds like I might be best passing on this one, and continuing to work on the deal on the 4-core 4,1. It's probably going to end up being about $500(plus some more money to buy RAM from OWC or Crucial, as it's currently at 3gb), but considering the "nothing older than a 4,1" advice it's probably going to be the best option. Of course, the early '08 MBP I'm typing this post from is running great with 3gb of RAM and an SSD on 10.9, so maybe I don't need to go nuts over RAM either(or at least keep it conservative and only go up to 6 or 8).
The biggest attraction to the 2,1 is the 30", so perhaps I'll see if I can work out a deal on it separately. Of course, even though as a staff member and adjunct instructor, talking the department into buying the Mac I
would want(probably a high end 27" iMac) is out of the question, but I think I could at least swing getting a couple of nice LED/IPS displays. As much as I've always lusted after a 30", a pair of 1920x1080 would actually give me a few more pixels of working area than a 30".
