buying used motor

$300 isn't bad at all, especially if it has an edelbrock on it. Figure the intake is probably worth around $100, so $200 for the engine. If it runs its worth it, as long as you're not worried about putting 100k miles on it. Even if it turns out to need a rebuild, you could probably get your money back parting it out.

The real question is, do you have a project for it? And does it fit with the goal of the project? Ie, I have a complete roller I'd really like to have run now and I'll fix it up nice later/as needed, vs, I really want this project to be a nice show car/race car/whatever.