mostly for home audio and video editing
I would suggest that you take a look at a typical workstation configuration.
I have done video editing on dual- and single xeon workstations (x5450 x 2, w3690/x5650/e5-2670 x 1) and I can see from personal experience that either approach is sufficient (for me). Typical cinebench benchmarking is quite indicative of real life experience with those configurations.
so i would recommend an older multi-core xeon, 16gb memory (ecc for lower cost), ssd + OK video as a starting point. a dual core configuration provides more upward upgradability in the future. the cost will depend on specific cpus picked. but a lga1366-based workstation should start at less than $100 used, $100 - 200 for a better cpu, $100 for ssd. a total of $500 will put you in business. lga2011-based workstation should start at around $500 used. newer ones will generally start at $1500plus.
There are lots of very cheap OEM E5 processors leaked through illegal channels from China and Japan, mostly from system integrators leaking chips unauthorized by Intel.
I doubt it. most of the E5 processors on the used market come from retired servers. I would pick x5650 if money is tight or E5-2670 as a starting point, as they offer really good value for your money.
I wouldn't get engineering samples as you may not be able to run them on your motherboard.
workstation configuration should be considered and don't waste your money on graphics -> unless you want to play games routinely on that computer as well.