Personally, expecting 600GB, of bandwidth, 512MB of ram, and Windows isn't going to happen for $30 let alone $20(coming from experience), so you will either need to low your requirements or raise your budget.
How much space/bandwidth/ram are you actually using right now?
Do you need a control panel?
Any reason you prefer linux of Windows? Is it simply because it's what you're familiar with?
Actually 128MB OR 256MB is enough for now, No I don't need any control panel, And yes I prefer windows since I know how to setup VPN servers for it, for linux there is a learning curve (to configure openvpn or some vpn software alike) that I don't have enough time and also need VPN server to be up and running by tomorrow.