That’s definitely going to limit your options. Getting something used will likely be your best bet.

If you want to buy new, and already have a 500-series rack, you may want to save an additional ~$50 and consider this: https://chambordaudio.com/products/ca-250-parametric-eq

No experience with that unit, but it would meet your needs and it’s the cheapest thing I can think of at the moment that would actually be a step up from what you’re currently using.