Should I buy my friends 5'7 Channel islands Al Merrick??

Hi i have been surfing for a couple of years now and up till this summer I have been using a 8'0 foam longboard and it was just too much i need to get a short board. So i ask people for advice and me not knowing much about shortboards i bought my tall friends 6'2 shortboard 18 9/16 width and 2 1/4 thick for $235.I am 16 years old and 5'5 i surf new jersey in the summer and rest of the year new york(rockaway and long beach) the 6'2 shortboard i have, i have been getting many comments that its not the right board to use for waves like 3-5 and its for monster waves like 8-10 or something.I didnt know when i bought it in the summer that it would be too big for me, i only got it because i was coming from a 8'0 ft long foam board and i wouldnt be used to a shortboard. After having it for 5 months my friend tells me its just ruining my surfing potential and shouldnt be using it on smaller than 7-8 waves. My other friend just told me he has a channel islands al merrick that is 5'7, 19 inches wide and 2 3/4 thick. its practically brand new and he just bought it 2-3 months ago and used it a few times. It has not 1 pressure, no dings, its white and has 1 fix. He got it from a local surfshop in new jersey that was a custom order for a stock dimensions thats around $800 but since hes my friend and wants me to improve much more and use a normal size board for all around waves hes going to sell it to me for $470 with good medium Future al merrick fins brand new in the package a leash and stomp pad included. and i want some advice if i should get it or what height board would be good for me. Some people tell me to get a 5'9 or 5'10 and i dont know what to do.

Here are 2 pictures the board with green wax on it that says Dane on it is my 6'2 shortboard and the other one is my friends 5'7 channel islands Al Merrick

 

its a motor boat too