I've got a couple of SPARCstation 4/5/20 and never had any trouble with the purple trim on them - it's all quite solid still. Perhaps sun exposure matters? The tabs on the front of the lid however - those are broken off on some machines.
As for which machine?
- If you want a SPARCstation 20 you'll really want to get one with at least two CPUs and a VSIMM to enable the SX framebuffer (the onboard 13W3 video port) otherwise there is kind of no point. Multiple CPUs and the SX framebuffer were pretty much the reason to get one of those machines over the cheaper SPARCstation 5 AFAIK. From what I've seen MBus modules and VSIMMs (and SPARCstation 20s themselves) are quite expensive on ebay so I wouldn't count on being able to upgrade one of these machines for anything resembling a reasonable price - my SPARCstation 20 will likely forever remain a bottom of the line uniprocessor machine with budget CGSIX graphics.
- The SPARCstation 5 is pretty much just a SPARCstation 20 without the MBUS slots (so only one CPU), no VSIMM slots or SX framebuffer, and one less SBus slot. It adds an AFX slot for the optional S24/TCX framebuffer which is sort of the predecessor to the UPA/Creator cards found on the Ultra series.
- And the SPARCstation 4 is pretty much just a SPARCstation 5 with no onboard audio (it's an optional card), one fewer hard disk slot, goes down to one SBus slot but has a lower-end onboard 8bit version of the TCX framebuffer (which frees up the one SBus slot for other things).
- The lunchbox SPARC machines are cute and conveniently small, though not fast. But then no SPARCstation is fast by modern standards. I quite like them. The MiniDIN serial ports on these use the same pinout as classic Macs making adapters relatively easy to find. Not all models have 10baseT so you may need an ethernet MAU.
- The Ultra series might be worth looking at too - most of these should still be supported by Linux if thats of interest. The Ultra 1 and Ultra 2 still use SBus expansion slots. The Creator models replace one SBus slot with a UPA slot for a special Creator accelerated framebuffer card and also bump the ethernet up to 10/100 and upgrade the SCSI interface too. So the Creator models are worthwhile even if the UPA graphics card is missing.
- The rest of the Ultra series use PCI for expansion, VGA ports (unless you've got a Creator3D card installed) so you can use a normal monitor and may have IDE instead of SCSI. The PCI slots mean you can get a SunPCi - basically a PC on a PCI expansion card to let you run Windows in a Window under Solaris. The first generation card (for the Ultra 5 and 10) has AMD K6-II at 300 or 400MHz, a VGA port (so you can use an external monitor instead), USB, Audio (ESS AudioDrive), serial and parallel ports. IDE drives and the floppy drive are emulated by the SunPC software. Kind of fun. Apparently the VGA emulation isn't perfect so for playing DOS games you'd want to hook up an external monitor.
Most SPARC machines need a special Sun keyboard and mouse, though newer machines (the Blade series and newer Ultras at least) use USB instead though ideally you'd still have the appropriate type 6 or 7 USB keyboard. If you don't have a keyboard plugged in system will assume you're running it headless and output everything to the first Serial port instead. So you don't need the special keyboard to use a SPARCstation - it's only necessary if you want to plug a monitor in rather than use it over the network.
You'd probably want an external SCSI CD-ROM drive supporting 512 byte sectors with appropriate cables and terminators if it's not a newer machine with IDE. The lunchbox machines of course have no 5.25" bay for an internal drive, and many of the 4/5/20 machines require a special low profile CD-ROM drive so if there isn't already one installed you may have a hard time finding one that will fit. All machines can network boot though so if you don't have a CD-ROM drive you can still get an OS on that way.
All SPARC machines of this era use an NVRAM chip that has a limited lifespan. These machines store their ethernet MAC address and Host ID in the NVRAM chip so if that's not working a lot of machines will simply refuse to boot. On some (but not all) models you can just replace the chip with a new one (its socketed) but make sure its an actual brand new one from a reputable supplier - not some "new" one from ebay thats been sitting on a shelf for years and has a half-flat battery. Alternatively you can slice into the side of the chip and wire in a new battery which is what I do as the replacement NVRAM chips are not cheap. If the NVRAM chip is still good check there is no firmware password set as bypassing these can be a bit of a pain if you don't want to replace the NVRAM chip.
If you go for one of the lunchbox form-factor machines (SPARCstation IPC, IPX, LX, XZ, SPARCclassic, SPARCclassic X) you will have to recap the power supply sooner or later. Preferably sooner - don't wait for it to stop working. If possible inspect the machine first and check for an unpleasant fishy smell or rust on the power supply cover as this could indicate the capacitors have already started leaking their corrosive electrolyte. Recapping these PSUs isn't hard so it's not a reason not to buy one (I've got two and would like more), it's just a (I hope) one-off maintenance task you'll have to do like dealing with the NVRAM chip. The job is much easier if you do it before the capacitors start leaking.
I can probably answer more specific questions or provide further detail on anything if required as this is all quite fresh in my mind right now. I'm currently going through my SPARCs trying to fix what doesn't work, then benchmarking and installing Solaris on what does work: