Selecting Suns is easy because there aren't many bad choices in the era you're talking about, but a little weird because the internal names and the package label names don't always match in obvious ways. Most of the "classic era" Sparc boxes are Sun-4 variants, with SparcStatons mostly being Sun-4c or Sun-4m and Ultras mostly being Sun-4u machines. The Sun-4* name is more important to knowing what you are looking at than the case badge. For example, I have a "SparcServer 20" that some previous owner installed a TurboGX (cgsix) video board in, so it's almost exactly a similarly-spec'd SparcStation20 with different badges.

Pre-SparcStation Sun-3 and Sun-4 VME based machines are quite a bit more exotic to source parts for in a modern context, and newer stuff are PCs (remember they did go and re-use the Ultra name for a family of x86 boxes a couple years later, so watch model numbers if you're trying to buy a SPARC Ultra).
SparcStations are a little more bespoke and workstation-y (SBus cards, SCSI discs) and Ultras are generally a little more PC-like (mostly PCI cards, ATA discs), but neither are particularly hard to work on these days since the common SBus peripherals aren't terribly expensive and SCSI disc emulators like BlueSCSIs have come down in price and up in performance. IIRC, in all cases you have to be kind of specific with RAM, some older machines use memory modules unique to the family and Ultras mostly take 168pin PC style DIMMs but are picky about the exact details.

IMO the SS10/SS20/SS5 Sun-4m machines are pretty nice to work with because they are still "workstation grade" high reliability parts but were made in HUGE quantities and are extremely modular within the family so it's easy to work on them and get parts/upgrades/documentation/etc. They also have 10baseT Ethernet onboard (careful about degrading your whole switch), while the older SS1/SS2 need an AUI transceiver.

Peripherals:

Remember that older Suns use their own protocol over MiniDIN-8 for keyboard and mouse and 13-W3 video cables. You'll need a suitable Sun keyboard (probably a Type 5 or Type 6) and mouse, and those can be expensive on their own if not bundled because keyboard people. They're not as bad as some of the more exotic and/or desirable to keyboard enthusiast bespoke keyboards, but still pay attention when considering a machine to buy. Video is a little easier because 13W3-to-VGA cables are a thing, (I have one of these with switches so you can configure for Sun or SGI or Next or IBM's particular signaling). You still need a monitor or scan converter that works with Sync-On-Green to accept the signal... most modern LCDs with VGA ports actually can, but the labeling is typically not very clear about that. Sun video adapters are generally a little more willing to negotiate video modes than some of the other workstations (eg. My SS20 has talked to almost everything I've plugged it into, my HP Apollo 9000/735 and its absurd CRX-24z video board will talk to the Dell P2314H on my real work desk and has spurned every other monitor I've tried it with).

NVRAM:

Most older Suns have a chip on the motherboard - typically with a yellow barcode sticker if it's original - which contains a small battery-backed NVRAM storing the serial number, the Ethernet MAC, and various configuration parameters, and a RTC (Real Time Clock). At this point the internal batteries on all of them should be presumed dead. The M48Txx line of chips Suns use were originally made by Mostek, who was absorbed by SGS-Thompson, who became STMicro. Ref for NVRAM chips. Once it dies the machine loses its machine ID and MAC address and such. Fortunately, they can be reprogrammed from OpenFirmware, either with original values read from stickers and the like, or suitable made-up replacements. There are a lot of surviving Suns hand-assigned MAC addresses containing amusing strings like DEAD, BEEF, CAFE, C0FFEE etc. as people have made up suitable numbers. Sun's factory MAC addresses have a 08:00:20 prefix if you want networking tools that notice that sort of thing to assume it's a Sun.

Generally there are 3(and a half) options for dealing with them:

1. Modern production compatibles are still available though you have to be a bit careful about model compatibility, and they're rather expensive these days, something like $25 a piece (eg. Mauser has a small stock of MT48T08s for $26.50+S&H ).

2. You can also grind an end and attach a 3.3v coincell battery holder yourself - some folks say you should always cut the old battery all the way out because there may be unwanted effects to having the dead battery in parallel with the good one.

3. You can crack the whole top of the module with the battery and crystal off and solder on a module with a replacement crystal and user-serviceable battery holder in place.

4. For rarely-used machines, you can just do the reprogramming procedure (in the first ref) at the OpenFirmware OK prompt by hand each time you start the machine, it will hold while the computer is powered.

It's not a huge deal, but it is a thing to expect to have to deal with.

Software:

Remember that the OS nomenclature is a little weird because Solaris started out being versioned on top of SunOS (eg. SunOS 5.1 hosts Solaris 2.1), and at they dropped the SunOS name then leading "2" from Solaris versions so you have Solaris 2.5->2.6->7->8. The Wikipedia version history table is straightforward enough to work through, and has decent notes on supported systems. You'll generally be between 2.1 and 9 on the era of systems you're talking about, and those are the ones that "feel" like old commercial workstation Unix with OpenWindows and CDE and whatnot - I'm partial to 7 as "peak Solaris" but I'm sure that's because I helped maintain a bunch of 7 boxes at one point, it's a fully mature SVR4 with all the commercial Unix-isms before it started to converge with the modern Free Unix-likes. Many of the usual suspects like Tenox and WinWorldPC have install media and/or software.

Edited to add from downthread:

Emulation:

If you don't want to fuck around with large pieces of aging hardware and just want to marinate yourself in a retro Solaris environment, the qemu sparc support is really good. Folks restoring Sun stuff with disc issues often do their installs via netboot from an emulated server. Adafruit even has a beginner click-by-click tutorial for spinning your own emulated Sun4m system.

I have a Sun Netra X1 sitting on the floor that I've been meaning to get rid of. One thing to keep in mind with the Netra and Sun Fire servers: they have a System Configuration Card in the back which holds their MAC address, NVRAM settings etc.. don't buy one with it missing.