If you're not ordering the new PC immediately, I'm pretty sure the new B550 Motherboards are less than month away from dropping? They're a bit less flashy, but they should be cheaper. I think they're supposed to be in the $150-$250 USD range? Maybe, when they release, X570 prices will normalize as well....
Given the weird current state of things, nothing is going to be a "bargain". Your best hope is
value, and it really is the slimmest of margins between the two sides, at the moment. If you did
anything on your computer outside of gaming (streaming, video editing, music production, intense spreadsheets, or programming), I wouldn't even include Intel, just out of principle. Their new 10xxx stuff makes no sense, and they cut features off the KF chips without lowering the price...
However, I'm not sure any of that really matters in your use case.
I'm not sure if you need all the cores and threads, or the bits removed from the KF chips. Being an iRacer, you'll need all the GHz you can to keep those frame rates up at Nürburgring.
Unfortunately it's not as simple as comparing GHz and picking the highest, so I'd say its probably so close it's a wash, but Intel still holds the edge on FPS for CPU-strapped games, like iRacing, albeit by the smallest of margins...
Intel
i7-9700KF; 8 cores/8 threads @3.6 GHz (4.9 GHz Boost), $390
i9-9900K; 8 cores/16 threads @3.6 GHz (5.0 GHz Boost), $520
z390 Motherboard: $250-$350
Recommended RAM Speed: 32GB (2x16GB) DDR4 3000, CAS 16, $160
AMD
3700x; 8 cores/16 threads @3.6 GHz (4.4 GHz Max), $300
3900x; 12 cores/ 24 threads @3.8 GHz (4.6 GHz Max), $431
X570 Motherboard: $300-$450
Recommended RAM Speed: 32GB (2x16GB) DDR4 3600, CAS 16, $180
One thing that Matt touched on, but it makes a big difference for one, over the other; Ryzen REALLY likes high RAM speed. You're going to be upgrading to DDR4, I'm assuming, so if you go Ryzen, you're going to want DDR4 3600 to get the most out of it, which is also going to add to the price. If you go with Intel, you're not going to see much benefit above DDR4 3000. Fortunately, again, we are currently in a weird time where there isn't much price difference between them, either way you go. Whichever way you do go, just make sure the CAS Latency is no higher than 16. The lower the number, the better, and 16 is the best I could find at those speeds.
And finally, PSU. Don't overlook this! Anytime something electronic fails, I worry a lot about the PSU. It is the heart of your PC, and the most likely source for any electronic failure that may occur in your PC, so I'd really recommend replacing the one you have. Using a
PSU Calculator I figure 750W should be plenty for your use. Whatever you get, just make sure it's "80+ Bronze" or better (Silver, Gold... Platinum if we're throwing all the money...)
