Hey, i just picked up a 1974 univox hi-flyer a few weeks ago!
I think $350 is completely reasonable. Mine was advertised at $400 (which is fairly common in this area), but i made a bass trade.
It's an incredible short-scae bass. The neck is phenomenal. The body is super light (i THINK they were made from basswood, but i could be wrong), so there's a little neck dive... but certainly not the worst dive i've experienced.
I've played Eastwood Hi-Flyers, Aria copies, Dillion copies, and O.G. Mosrites. The Univoxes are consistently the best of the bunch. The Eastwoods are cool... but there is such a wide range of good and bad, they're kind of a crap-shoot, unless you can play one first.
I say go for it!