I'm in the same boat. 6 year old i7 4770k machine died (suspect mobo) but I'm not willing to risk buying a used LGA1150 mobo to test my theory.
I was looking at an i9 9900k build but that was coming in at over £800 for a mobo+cpu+ram. Then I looked at an i7 9700k build, but that was over £600, until I stumbled on some video reviews of the i5 9600k. Base clock is 3.7ghz, but can easily be pushed to 5ghz (apparently). Build price is under £400, so will probably go down that route.