A powder tank will be no good as your main tank, it would only be suitable for a buffer tank. For a basic system you will need;

CO2 Tank - Cost about £35, standard is a 600g bottle
Regulator - A modified welding regulator is normaly fine, but dont modify it yourself, get someone who has done it before.
PRD - safety valve thats lets the gas out if the pressure goes over a set amount, you should have a 1000psi one on the high pressure side aswell but these are now near impossible to get hold of
Valve - to control the flow of gas into ram cost anywhere from £40 upwards new
Ram/Cylinder - size/cost will depend on what power flip you want.
RC switch - such as an electronize switcher to fire the valve from your RX.

Check out;

http://howto.killerhurtz.co.uk/pneumatics.htmhttp://howto.killerhurtz.co.uk/pneumatics.htm