Hi,
I'm rendering with Redshift, but hardware wise, same goes for Octane. Go with 2-3 RTX 2070 Super or RTX 2080 Super cards (or one RTX 2080 Titan, and add second one later), high IPC cpu, no need for bunch of cores, speed per core is more important, and 32-64Gb of ram.
Of course, if you have higher budget, you can add more cards or more ram, but this is good for start.
Dane