2. The input voltage is 12 V, the output is 230V AC. The currents will be in the inverse ratio, so 300W at 12 V = 25 A, and at the output 230 V the current will be about 300/230 ~ 1.3 A. You need to use very thick wire for the 12V and there will not be enough room on the transformer to wind the secondary with this gauge wire. Its also a pain to wind with this thick wire.
3. Primary wire must be able to handle 25A without getting warm, so as the kit is handling 300W,the losses in the wire should be less then 1% total, so allow .5% for the primary and .5% for the secondary. So impedance of input circuit = 12/25 ~ .5 ohms so resistance of primary wire should be .5 X .005 ohms = 2.5 milli ohms. So work out the length of wire required for the primary and look up wire tables to select this length with a better then 2.5 milli ohm resistance, making sure it can handle 25 Amps. repeat same procedure for secondary.
Frank