[NAME].Simpson  1) According to UserBenchmark, there are several systems benchmarked with the i9-9900 so I would say it should work. 2) The Dell cooler for the i9-9900K should work and at least it fits.
My current processor:    Intel(R) Core(TM) i5-8400 CPU @ 2.80GHz, 2808 Mhz, 6 Core(s), 6 Logical Processor(s)  According to the following post, the 9th Gen i9-9900K will work with my XPS 8930. [URL] 1) Can I use the locked/no overclocking version - Intel Core i9-9900? 2) My second question is: What's a good cooler for the upgrade?  It needs to fit in the computer case so it can't be too thick. Thank you for your help.
XPS 8930, i9-9900K and cooler upgrade