Another hobby? Some Sun swag instead? Biting the bullet and buying what you want?
I mean the specifics of this isn't something where what works for me is going to work for you. And vice versa. It's your nostalgia, your money, and your time.
To me, it looks like a tinkering hobby. There are lots of things to tinker on. Sun workstations look like one where getting stuff to work smoothly or run at all is a rabbit hole of eBay searches without the wealth to just buy whatever you want.
It's premium GAS or parts collecting with finding replacement custom ASIC's and soldering bad caps on disc controllers. Sun workstations aren't C64's or even Amigas.
What I ended up doing was buying mint Java workstation since it has cool Java logo on front and nice big Sun Microsystems on side of case. Plus it has a roomy case that isn't too hard to retrofit. I am gutting it and putting in modern parts.
That said, if you want the retro-computing vibe, for me the peak of Sun's industrial design was the SPARCstation 1/1+ pizza box, with matching style mono CRT, and the matching external SCSI DAT drive, CD drive, and extension hard drive. To me, they're superior industrial design to any of the Apple machines, and are as iconic as the NeXT cube.
From a technology point of view, the sun4d machines (a SPARCserver 1000 is the more practical) are awesome. Load the SS1000 up with 4 CPU cards (8 CPUs), and enjoy what was some of the best performance of its era. But beware, they need a lot of watts and weigh a ton.
[0] https://dcomcomputers.com/sun-e450-enterprise-450-2400mhz-1g...
Sun's own JVM worked differently under Solaris & Windows ...leading to the disparaging acronym "WODE" - Write Once, Debug Everywhere :)