how far can these device could be connected with its internal antenna if put on an outdoor plastic box? with a clear line of sight
100 MTRS.
how about 200 meters?
and perhaps i have to remove the indoor plastic box?
can go where? depends on the clients. no matter what AP you use, laptops will have a limit of 100m because of their small antennas and weak transmitters.
go to another RB751U/G 2HnD with same setup(inside of an outdoor plastic box). how far would it be? thanks for answering 
so can i go on with this? i need a confirmation ASAP, so i can start ordering them.
We have not done any tests like this, but wireless link calculators suggest that 2Km should be possible in absolute ideal conditions and no interference.
i see. that is good to hear. perhaps the question is, how much signal loss am i gonna get for the plastic outdoor box? the plastic box is pretty thin, because i’m gonna use the cheapest one 
what about using SXT Instead ?
the site, is only about 200m radius. RB751U/G also has level 4 license, thus functioning better, especially when adding more client on the site. SXT could only handle 1 client.