$600 doesn't sound like you need to only buy one thing to me. I would probably buy a manual fast 50 and an auto-flash, and then think about what to buy for the left over money.
I would recommend a 50 with A mode, which gives you full automation except for manual focus. The A50/1.7 should cost less $100, though I don't keep track of prices. And an auto-flash should cost about $20, or a little more for a powerful one. ("Auto" in this case doesn't mean what you might expect, a modern flash that talks to the camera about anything more than "flash now" will cost more.)
If you feel you need auto focus the FA50/1.4 should still be well within your budget.
The only problem with older flashes is they can have a high trigger voltage (which can damage your camera). Check this before purchase.
Both these suggestions require you to learn new things to use, but if you don't want new experiences you can stick with what you have.