I use a Mac Mini as a project server with Windows/Mac/Linux workstations/laptops. It's completely seamless as it all comes down to just TCP/IP communication. For a project server without media, any recent quad core Intel i5 or better on any platform should suffice. Depends on the number of clients of course, but you're not likely to saturate the CPU or network with just project data unless you're a large company. I know the old DaVinci Resolve 14 Configuration Guide recommended a dual core 3GHz Intel i7.
If you don't want to mess with the database server installation/configuration yourself, it helps if it's a system where Resolve can be installed (it doesn't have to run well), but it's not a requirement.
If all your workstations are Windows you don't even have to set up mapped mounts in Resolve as they will all use the same path to the NAS.
Technically the project server can be any machine that is always on, but I like running it on a small standalone Mac Mini or Intel NUC. If anything should happen you can get a new one on short notice, perform a complete restore from the backup and be up and running again very quickly.