Picking 486 as your PC #2 is fine choice. I was going to recommend 386, but it's such a personal decision that there's really no wrong decision. Everyone has different motivation for what to setup/keep/use. For me it's a combination of emotional attachment to something I had or something I always wanted. Your choice of IBM PC 5155, 486, P3, covers nicely different eras. I currently use primarily 386/33 and Dual PPro200. Bunch of other PCs are in a closet. If I wanted to be purely rational (while running on old HW), I would probably just use my single P4 machine which has ISA slots, can be clocked down to 286 level, multiboot and run smoothly virtually anything booting into DOS, Win98, WinXP.
Sinclair ZX Spectrum 48k, 48k+, 128k, +2
Amiga 1200, 68030/40mhz
386DX/33, ET4000, SBPro2, MT32
Dual PPro/200, Millennium II, Voodoo 2, AWE32, SC-55
etc.