with your budget constraints i would go for a vintage Seiko diver. they have a few big , chunky and iconic diver models that can be had for <$500 and are excellent quality.. unless you are strictly into swiss-seiko made some of the absolute best working tool divers in the late 60s to late 70s. you might try searching eBay for 'vintage seiko turtle' 'tuna', or just vintage seiko diver. if you want a decent swiss for that price i would try to find a vintage SICURA. They have some sort of association with Breathing and are well loved by quite a few enthusiasts. i think you might find some for under 5. everything else i can think of thats decent is over budget. good luck
*Oh...I just thought of a great , near iconic and totlally respectable vintage diver: Look at Boluva Snorkel 666 automatic diver and you can find a few pretty nice examples for just under $500. There are newer Boluva divers with assertion movements that i don't favour. But the Snorkel 666 come in black dial/blk bezel black dial/red&black bezel and the much more rare (but not that expensive) orange dial snorkel.