This may give you some ideas:
Careful of Xeon hardware, TheClue had some issues: Issues installing+booting Haiku on a big-boss desktop with W10
You probably want to get a known working Mobo and a processor with known working graphics… I’m looking and testing out a KGPE-D16 system soon a bit dated but still beefy and easy to add a ton of ram to inexpensively. A thing to watch out for is quirky USB3 chipsets. Ryzen doesn’t work well yet but it’s being worked on I can boot my Asrock ITX board + AMD Ryzen 1700x with the 4GB ram limit enabled and and other than 4GB ram limit it’s super fast. Vega graphics do not work so I set it to failsafe graphics, I culd install an RX560 and it would be fine though most likely. I would suggest getting a Radeon Northern Islands card as that and Intel graphics up to Haswell is what will work the soonest when waddlesplash gets going on the 3d acceleration work. Vega graphics doesn’t work yet.
That said Radeon GCN cards probably up to Polaris RX580 should work, definitly don’t buy a card that is blacklisted here https://github.com/haiku/haiku/blob/abb59d7351c7ddb50c63c40430a82d94fa61917a/src/add-ons/kernel/drivers/graphics/radeon_hd/driver.cpp