No extra cost? Don't buy a gun!! takes space and wastes £100!! thou it could have some benefits as well..
But maybe getting a cleaning rod and some silicon spray and clean the gun you're using.. as well as always adjusting the hopup *if the wheel is there XD*
I don't know rental works, if you get to choose your gun? but if you find one that you like, maybe you could ask a marshal if it's ok you put a small dot with a marker inside of the mag well. That way you can always try to get that specific rental gun. meaning you will only clean the barrel of the gun you will be using next time, and that you will be rewarded for using it with care, aswell as you knowing that you got a gun that is somewhat consistent. Could even get an optic sight and zero it for that gun, if they have RIS.
for sound reduction, you can warp some material around it. you could also make a DIY slide-on suppressor at home (you will only have to use primitive materials such as pet bottle plastic, cardboard, electric tape, duct tape, cotton, material, etc)
XD