forget the imac for 4k
Mac Benchmarks - Geekbench Browser click on the 64-bit multi core tab
from the beekbench benchmark, the older mac pro with the top of the line cpu's are somewhat close to the new mac pro. by the time you upgrade an old mac pro with workstation quadro or firepro, it might be close to the same cost as the new mac pro. if you go with non-workstation graphics cards the old mac pro may be a good value.
i don't use mac, so not sure which nvidia issue you are asking about, and if its been fixed. maybe someone else here can answer that.
for the video card(s) in the old mac pro, the quadro k4200 wouldn't be enough. if you are set on workstation grade video cards there is a mac version quadro k5000, but you may want to look at faster cards like dual quadro k5200 or dual amd firepro W7100, or a single quadro M6000. desktop grade cards from nvidia or amd will be alot cheaper and work just as well with premiere. a single gtx 980 ti may be enough. if you need 10bit out for a monitor, you can add a video out card from blackmagic or aja etc. supposedly switching video cards to a non-mac version will remove the mac boot screen. you can either keep the old video card around to swap out if needed, or look at getting a mac bios flashed card, like from here. MacVidCards.com - Home
also with the old mac pro, you will want to make sure the power supply can handle the extra demand of the video card(s). also check the available pcie power leads to plug into the video card(s) and check the video card you plan on purchasing as well. some cards like the quadro m6000 want a 8-pin power connector, but come with an 2x6-pin to 1x8-pin adapter. the amd firepro W8100 uses 2x6-pin plugins, the quadro k5200 and amd firepro w7100 use a single 6-pin plugin. so depending on what's available, you may be limited to certain video cards. Do I Need An Additional Power Supply? - MacVidCards.com
Quick question: What's your budget?
RE: How many cores are optimal for Premier?
Premiere should use every core that's a available. So, as many cores as fits your budget. After that, the rule of thumb is two GB of RAM per core.