If you only want SER and GSA Captcha breaker to run on the VPS then 4GB ram and 80GB hdd is the minimum you will need. The more cores the better but as most provider will sell you packages when you choose the adequate ram and hdd you will get 2-4 cores of cpu too.
You may run other low resource requirement tools on this vps depending on how oversold the server is.
Depends on the size of your project but I worked with 1tb bandwidth without problems.
The further you can get from seo vps providers the better vps you can get for your money. Especially if you can find a juicy discount. You always have to use a proxy though and can't really ask for help when one of your tools messes up something.
Comments
16 GB ram
Unlimited bandwidth
These are the facts I considered when buying a vps. Good luck!