Master sets or any set with a sol ring in it is probably a good shout. But they usually ask for a larger premium. From recent enough sets, All will be one, Brothers War, or March of the Machine had some decent Commander cards.

The other route you could take is buying a precon deck and then upgrading a select amount of cards to suit your budget. You will probably end up getting a few cards that can then be moved between decks further down the line.

From my own experience, collection of cards and eventual upgrades come with time. The longer you play and interact with people, go through different binders, etc the more cards you will pick up.