You can do this on any Intel/AMD box you may already own - simply run a virtual machine if your main OS isn't Debian.

See here: cross compiling.

If you want a dedicated box: I have a Minisforum UM780 dedicated to building WebKit and it does a good job. You don't need something that fast or expensive though. I'd totally get a Minisforum AMD NUC or a similar box from Aliexpress. At 500 eur you should be able to get something more than enough for any cross-compiling needs. I'd say 8 threads and 8GB ram would do just fine.