How do I combine a variable and characters into a string with Arduino?
You're thinking backwards. Instead of building up one string and comparing it to another, you should instead be splitting down the string you are being sent and dealing with the different parts of it separately.

Say you are being sent the string 32,read\r. Instead of making another string 32,read\r and comparing it to what you have received you should instead be splitting the incoming string up. 

That way you then have a string containing 32 which can be converted to an integer, and a string containing read. It now becomes very simple to do your comparisons without messing with building strings in RAM wasting precious memory.