I found yet another Dillon 450 on eBay that is within' your price range. It's $220.00 plus shipping. That gives you a total of $250.00 in the press and leaves the other $100.00 for the upgrade.
Dillon RL 450 Upgrade Frame Change Kit: Reloading Machine Upgrades & Updates

That keeps the press near the $350.00 limit you originally mentioned. It might go a little over, but not by much.