Wich upgrades,

Im gonna buy the SRU pdw-k kit in the future. So what is the best road to go on upgrades. Am i able to use any hopup unit whitout taking it from the kit? Can i dril a loki hole in there? So wich upgrades should i put in this thing?

Im gonna buy the SRU pdw-k kit in the future. So what is the best road to go on upgrades. Am i able to use any hopup unit whitout taking it from the kit? Can i dril a loki hole in there? So wich upgrades should i put in this thing?