Question

please when building an inverter, what should I consider most in buying my mosfet? is it the ampere or volt or wattage of the mosfet?

Answer 1

It always depend on application, and the limits of the device must match it.

Answer 2

For small projects, the maximum drain-source and gate-source voltage rating at room temperature is enough to be taken into account(usually 5V ).
But while designing a NOT gate to be built inside a processor etc, along with the above conditions, factors like maximum switching speed, time delay,ON resistance between drain and source(for noise margin), the amount of current it can source/sink to drive other inputs, temperature range,CMOS/TTL compatibility are considered.