If it is 700mm blade heli running 4 blades you want around 1000 rpm at a guess?
700 helis usually carry around 1000 - 1800g worth of packs and anything from 6s to 12s, the choice is yours.
The motor KV rating is how fast it spins per 1 volt so once you know how many Li-Po cells you will run in series (6s, 8s etc.) you can work out the rpm of the motor at 100% throttle. Divide the motor rpm by the gear ratio (main gear divided by pinion) and that's roughly the head speed at 100% throttle.
Electric helis can be run at 100% without governing and are most efficient between 80-100% throttle.
Li-Po packs are best discharged only 80% of their capacity for long life so taking 4000mah out of a 5000mah pack for example.
To figure ESC size you can work out your average current draw once you know your battery configuration. So if you will be using 12s 5000mah (2x 6s 5000mah packs in series), using the 80% rule above that's 4000mah per flight. At 1000 rpm scale flight you will get a good 10 minutes flight time. 4000mah divided by 10 is 400mah per minute 24,000mah per hour or 24Ah (24 amps), this would be your average current draw. You should then double this figure to be on the safe side and use the next size up ESC so in this case a 50,60,70 or 80A HV (High Voltage) ESC would be adequate.
If you let us know the gear ratio and blade size we can suggest a few setups.
60% of the time, it works every time!