Whilst it's certainly possible to grind al. alloy, most people with a surface grinder keep their machines set up to grind steel, the grinding wheels are different, and the magnetic chucks don't work too well. I would think that a suitable manual mill, like a Bridgeport, would do the trick, using a fly cutter, setting it up would be a challenge and would be where most of your money would go. It doesn't require anything glamourous, just competent.
Getting a piece of plate glass, and lapping it would be the easiest, and cheapest, and being a cheapskate would be what I would do.
cheers
wakeup