Maybe ... none? Or at least, without being the most expensive Mac beast available in most cases.
I don't know if you understand what you're expecting that hardware to do ... but h.264 mirrorless is all highly compressed long-GOP, and doing it in 4k just compounds the troubles. It is incredibly demanding of the CPU for playback, and while a basic video player can get by as it places no other demands on the CPU, an NLE by nature is itself extremely demanding of the CPU and the rest of the computer.
Lumetri ... well now, that's one of the more demanding resource hogs of PrPro. So you're adding that in. And if you really want to do much with Lumetri, as I tend to ... you need to stack them at least occasionally, further compounding the issue.
Except for time-ramping and merged clips, the "native" proxy process in PrPro is pretty slick ... make sure it's the Cineform built-in preset (no clue why anyone would use an H.264 format ... long-GOP! ... for proxies, a waste of time) and you can quickly set a keyboard short to cut forth & back to proxy/original media. Paused, go to original ... for playback, switch to proxy.
For time-ramped/merged clips, transcode those to either Cineform YUV or DNxHR or ProRes and replace your original media on the timeline with the transcodes. See that your previews are also a Cineform, DNxHR, or ProRes format.
Do that, and you can edit that media pretty slick with most Macs.
Was there a particular reason you wanted to avoid creating proxy files and/or transcoding to Apple ProRes422 (LT) or higher?
You'll get good performance from any Mac thats 2012 or newer while using Thunderbolt or Fibre-channel storage.
Perhaps look at your budget (do you want to spend $1,800 or $12,000?) and then buy a Mac with the most RAM, fastest processors, and largest internal storage that that will get you.
If you're willing to consider a PC to replace your Mac there's a myriad of options to create a system that will handle 4k smoothly and most of those options are at a lower price point. And I don't think it can be overstated that a workflow without proxies is simply easier.
I've definitely come to that conclusion. So tomorrow I have some parts coming in to build a PC: Ryzen 1600 / 1050ti / 16gb RAM / 256gb NVME / 500gb SSD ... Not the best GPU, but by all accounts, it should work without Proxies in at least 1/2 resolution. I guess we'll find out!
Sounds like you've figured it out. Good luck with the build. If anything, you may need to bump your ram to 32gig but you're certainly on the right track.