If I had $5,000, forget the Amiga 1200 - I could actually AFFORD the Amiga 4000 for a change, perhaps with one of those rare PowerPC accelerators thrown in and a good RTG card to sweeten the deal, and on top of all that, an Apollo Vampire 500 V2+ accelerator that I bought my Amiga 500 for.
I might even have enough left over for a Sharp X68030 with 68060 accelerator, a Fujitsu FM Towns, and an NEC MultiSync XM29/XV29/XP29 family monitor to hook up to all of them. 27" of surprisingly-good-for-a-shadow-mask CRT goodness, handles 15 and 24 KHz without complaint, NTSC and PAL alike, also great for light pens/guns for systems that support 'em. They're probably the only non-aperture grille CRTs I'd go with, just for the size and retro compatibility.
If I still have some room left in that massive budget, it's SGI time - particularly if I get my hands on something with N64 or other console devkit hardware in it, like the renowned SGI Indy find that had the Turok source code on it.
There's no need for me to blow that kind of money on retro IBM PC-compatible or Mac parts; I have just about everything I could want from those two categories already, and if anything, need to offload my excess Mac stuff before someone else throws it out (mainly IIcx, iMac G3 350 and possibly iBook G4 1.42).
Should my Power Mac 6500 give up the ghost, though (I just heard it making a very unsettling noise like a bearing gave out tonight, though it's still fully functional), it's getting replaced with a Power Mac 9600 - a whopping six PCI slots without any garbage integrated graphics to detract from that, all the legacy interfaces (including a floppy drive that can read DD 400/800K Mac disks), ludicrous RAM capacity for a system of its age (more than a Beige G3!), and G4 CPU upgrades. The best 68k Mac may be a classic Amiga with RTG and ShapeShifter or Fusion, but no ADB = no Thrustmaster FCS support, for starters. (You need a Mac FCS with associated WCS and PC gameport rudder pedals if you want analog throttle and rudder in Flying Nightmares or A-10 Attack!/Cuba!, to name a few. Emulators don't support this.)
Also, there is one IBM computer from that era I'd consider setting space aside to own: the IBM/Sega Teradrive, one half 286 PC, one half Mega Drive/Genesis, and both halves can talk to each other, like a glorified devkit.
So yeah, it's mostly Japanese computers and the one American computer line that saw more success in Europe than its homeland, where Commodore had every right to steamroll IBM and Apple with superior technology but utterly failed to do so. It's quite telling that importing an Amiga 500 over from Germany was more affordable for me than just buying one here in the States, because the only people who know that the Amiga exists also knows what it's worth on eBay!
As for components, I can't think of much left that I don't already have and isn't in the "overpriced" to "unobtanium" range yet.
A Voodoo5 5500 PCI Mac Edition would be nice for my MDD G4, so I can run an ATI card with Core Image support for OS X Leopard in the AGP slot without fear of extension conflicts under OS 9. A second one would be really nice if I ever got my hands on an Amiga 1200 or 4000 with some kind of PCI busboard for RTG.
A Forte VFX1 setup with the VIP card and whatever graphics card still has a fully functional VESA feature connector would be one hell of a way to relive my nostalgia for Quake in '90s VR, though I mainly want to play System Shock 1 with it, which can't be done on a Virtual i-O i-glasses! VPC setup. I could entirely forgo this if someone made a VR fork of DOSBox specifically designed to emulate '90s VR systems (VFX1/i-glasses!/CyberMaxx) using a modern Oculus Rift or HTC Vive setup.
Actually, speaking of VR, why don't I treat myself to one of those Virtuality systems mentioned earlier? Or, while not technically VR and just a fancy sim cockpit, I could use some of those Virtual World Tesla pods - the kind that ran pre-Firestorm Battletech (read: NOT just MW4 ported to an arcade cockpit) and Red Planet. I never got to play Red Planet while arcades in my area still had those pods...