I mostly ride early to mid-80's frames that have been modernized in terms of components, so I understand the inclination for the type of project you're looking to do, however, I think [objectively] that there's better options out there. The Soma Fog Cutter comes to mind if discs are an absolute requirement. Was the Space Horse disc the All-City you were thinking of buying? For me, the cool factor could be addressed by getting custom paint/powdercoat and doing what you want with it.
I like where your head's at and I know nothing about braking forces, etc. If you're seeking modern benefits like discs, I think a modern frame is the way to go. I've been OK doing my modernizations with long reach caliper brakes and decent pads. I fit 32's with fenders and get to customize the paint or keep it old school.
To address your actual question... my, hands down, most used bike is a resto-modded and repainted '83 Trek 620 (?) which I find to be versatile as can be. Perhaps not suited to cross races, but that's not likely your main goal since you're beginning with the idea of using a 30 year old bike frame as your starting point.
Either way, good luck and post pics if you end up doing it.