Unfortunately there is currently no official BSD release featuring a KDE Plasma desktop out of the box.
If you don't mind using MATE instead, go with GhostBSD (official release). The Xfce community spin is clearly not of the same quality. Never tried the Plasma version, but since it is not even listed on the website, I would be rather suspicious.
HelloSystem is really promising but it is still in development stage so I would not recommend it yet to somebody who doesn't like tinkering. It doesn't use Plasma by the way, but a custom interface of its own.
Otherwise, installing nvidia drivers and Plasma on regular FreeBSD is quite easy.