I doubt people buying used Macs are too concerned about CPU generation, as if they did, they surely wouldn't buy used. They are money-strapped people who want a Mac, but can't afford the brand new ones, and are ready to compromise on something to get a lower price tag.
And MacRumors doesn't represent what most Mac buyers are, i.e. people wanting a machine that "simply works". Haswell or not, I doubt there will be a revolution, rather than an evolution running the latest OS X on any recent CPU.
If you're in for a few months only, I'd say buy the cheapest Mac you can get, provided it has a good look as it will count a lot in retaining resale value. Bonus points if it still has warranty (stock or AppleCare).
Or if you really don't mind about resale and comfort, any Windows PC would do the trick.