What are currently good buys for a budget, used workstation purchase, please?

I have a friend for whom I built a Dell Precision T5600 dual processor machine about five years ago. She's a musician who doesn't have a large budget (so we were fishing in the "what's cheap and available" pool), but she DOES have a closet hardware fetish, hence the desire for a workstation.

Unfortunately that machine is now refusing to boot (POST error code points to a memory issue), and having removed all the memory, and testing various single pairs, one at a time, in various RAM slots, the machine just wont get past POST (Dell believes the MB is faulty). I've looked at getting a replacement motherboard, but a couple of them also seem to have dead RAM slots, so I'm thinking it's time to part that machine out and get her updated to whatever we can find under about $600.

I'm seeing some dual processor HP Z6 G4 and Dell Precision 7910 machines on eBay in her price range, but I'm wondering which other models I should be looking at. She currently has 128GB of RAM and is keen to keep things at that level (of course she is!!)

* When do machines tend to come off lease contracts - when are there market inventory spikes? End of quarter, or end of year? * How do Lenovo's Thinkstations compare to Dell & HP? Any other brand I should be considering? * Given the low budget, should I just find a decent, newer single processor PC which may outperform a dual proc. workstation from three to five years ago? (I've not kept up with the market for these kinds of cross-departmental comparisons)

Any other pointers gratefully received, thank you.