If you link to the site you are looking to buy this off, it may help judge it better.
But a 30% overclock is almost certainly a (deliberately) misleading number. It won't be 30% extra performance, as it won't be 30% over the CPU's own boost. In fact, 30% more than the 9900k's 3.6GHz base clocks is 4.7GHz... which is what the CPU will boost itself to on many motherboards anyway even under 8-core load. $100 to turn on / not switch off 'MCE' is absolutely outrageous!
Moreover, even if it was an overclock, you don't need to have someone else overclock your PC, you can DIY it, relatively straightforwardly. You just want an really good CPU cooler, and a better motherboard than an Asus TUF. Or, as above, leave it at stock frequencies.
Before you drop $500 on an i9 9900k, note that Intel are about to release new CPUs
including the i7 10700k, which will be better than the 9900k, $100 cheaper, and come with better motherboards.
if it's not urgent, wait a few weeks and buy the newer CPU and Z490 mobo releasing with it.