The marlin XL7 comes in 25-06, so if you're set on spending your $500 now, that's what I'd suggest. I'd prefer a 25-06 to a .243 by far.
But what I might also suggest is but the $500 in a safe spot and save up the next $200-$300 you need to buy a better gun! Like and FN model 70, or a Tikka.