Well, you're asking in the C&V forum, so you probably know what kind of answer you're going to get, right? The fact that you can do your own work tilts the scales very heavily in favor of vintage IMO, but it also depends on how patient you are and what your local market is like.
Assuming you're going to pay the retail price of $600 for the Space Horse, your point of comparison is nearly any vintage bike frame on the market. Let me show you a few vintage frames I've bought in the past year to give you an idea of the range of what's out there.
1987 Pinarello Montello -- $600
1982 Trek 614 -- $150
1986 Centurion LeMans RS -- $40
I paid about the same for the Pinarello that you would for a new Space Horse. It's not the kind of frame you're looking for, but I'm sharing to give you an idea of what a vintage frame in that price range can be like. When new it would have been painted, but the bare chrome when I bought it was in fantastic condition. Pinarello (whether justly so or not may be debated) is a brand that draws a premium price these days and the Montello was their top of the line frame in the mid to late 80's. It's got Columbus SLX tubing and came with a headset in good condition.
The Centurion isn't quite the other end of the spectrum, but it's about as low as I'd go unless I was just wanting something to tinker with in the garage. It has Tange 2 tubing (not top of the line, but not bad) with a Tange Mangalloy fork (kind of low end). It was about in the middle of Centurion's offerings in 1986. The paint on this one is badly chipped all over the place and the frame has a few minor dents. It came with a headset, Shimano HTII bottom bracket and mis-sized seatpost. This also isn't quite what you're after (it wouldn't take more than 700x28 tires), but it's close and you could likely find something of this quality in this price range that did have the characteristics you're looking for.
The Trek, I think, is just about exactly what you're after. It has a Reynolds 531 main triangle (excellent stuff) with Ishiwata stays and forks (above average but below 531). The paint was in excellent condition, though I can see places where the previous owner touched it up (seen by texture, not color mismatch). There is some minor damage on the chainstays from an over-zealous kickstand installation somewhere in its past. It came with a good headset and bottom bracket. This frame takes 27x1-1/4" tires with plenty of clearance. I haven't tried it with 700c wheels, but 35mm tires would probably fit. If I went down to 650B (a popular choice in rando circles), I'd be looking at 38's or maybe even 42's. It has all the braze-ons I'd need for modern components, fenders and a rack. In my mind, for $160 this frame kicks the Space Horse's booty pretty hard.