Hello there;
I am currently using 380T brushed motor with a 25A built in ESC-receiver unit in my modelart sandstorm truggy. I want to upgrade it to brushless motors. So what exact constraints should i see in choosing brushless motor( what kv's or what type i should use). Do i have to upgrade the ESC to a brushless one? Of How many 'A' should i use? I am using 25c 7.4 V li-po battery.
please help accordingly

You need to upgrade the ESC if you want to switch to brushless fr your current brushed setup.
380T is the size of the motor, so you can go for any similar size 380T in brushless Inrunner.
You can use 2S 25c+ lipos easily with this setup.1800-2500mAh shld be great weight to power ratio. Try to keep the buggy weight as close to original as possible 

If you chose to go extreme, which I personally think isn't that great idea for sandstorm model
You can even try 3S lipos with above setup, but u need to add motor/ESC cooling fans.
540T is the next bigger motor option,used in  1/10-1/12 models. That would be an overkill for its weight n power

Thanks buddy, but I can't find 25c 2s lipo ;that too 1 800mAh....  JH is also not having it.