For the brush sizes I use 3/0 for detail and 18/0 for very fine detail. You'll really be able to eyeball the point fine-ness thou so just examining the bristles closely will be your biggest help there.
As to metals I'd recommend Iron-breaker for your silver and Gehenna's Gold if you want to do Gold.
For Primer any acrylic based primer should work, I've been using Krylon's basic primer which you can pickup for cheap at any Supermarket that sells paints (wal-mart etc.)
Also I would highly HIGHLY recommend picking up some "Lahmian Medium" made by GW with your paints. If you thin them in a roughly 1:1 ratio (play around with the ratios to find what you like best, I use probably a 2:3 Medium to paint mix) it will help you to get a greater range of colors and for your paints to go on smoother.
All that should run you (if you just purchase the paints you mentioned, and if the prices where you purchase fromare similar) about $30-35 USD for a few brushes, primer, 5-6 paints (including the medium, I cannot stress enough how much better that makes the paints look). Hopefully that's within your range
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