Never use resistors for controlling power, that's just waste. PWM is used for controlling power. The duty
cycle sets the average power without having any unnecessary waste heat to git rid of.
Firstly to control a MOSFET directly from an Arduino requires a logic-level MOSFET, this is mandatory unless
you want to level-shift to 12V.
MOSFETs are picked by max voltage and on-resistance. The on-resistance determines the dissipation
for a given current load.
You can switch 5 heating elements in parallel if you want, that's simpler. Is each one 50W, or the total 50W?
What is the resistance of each heating element? (ie have you calculated that 50W correctly)