The NEC PC-98 series are 16/32-bit computers from, well, NEC. It marked a shift away from the Z80-based PC-88 to the x86 architecture. Just like their predecessor series, they were known for their unique sound hardware by YAMAHA, and games kept the systems popular. They were likely never released outside Japan in the series lifespan, except for the APC III.
I have not owned a PC-98, but here are my rough estimations if you want ReactOS to run on a PC-98.
According to the Supported Hardware/CPU page, you can only run ReactOS on i586 and above, so that means you can run it on any one of the models with a Pentium CPU, or the last model with a Celeron@433 MHz (the latter probably being the recommended).
The PC-98 series use a BIOS that is completely different from a Standard PC BIOS, so a new HAL will likely need to be made.