Is it ok to use a 30 amp esc with a motor of 18 amp max current? ?? with a 7.4 v lipo of 30c ??

or should I buy a 20 amp esc

As long as the source (ESC) is of greater capacity than the sink (Motor) it should be fine, because the motor will only draw as much as it needs.

As specified by rajasachdev.. The motor will draw only the amount of current specified even if you add a 40 or 50 a esc to it
The more the A of esc more the heavier it is so people prefer buying 20 to 25 A with such a motor
P.S the esc should be at least 5 A more than the amount drawn( learnt this lesson when I burnt 2 of those)