Power resistors equivalent thermal model: Interpretation of datasheet & 0°C terminal of power source
I'll try an answer.
I'm confused by your first question but let me try this,

So in doing this type of modeling there is the following analogy.


Temperature (T) = Voltage
Heat (power) (W)  = Current
Heat capacity (J/K) = Capacitance (coulumbs/volt)
Thermal resistance (K/W) = resistance (V/A)








simulate this circuit &ndash; Schematic created using CircuitLab

So I usually make my temperature reference 0 K.  (You could make it 0 C, but it gets confusing if you ever want to work at temperatures below zero C.)  The temperature reference like a voltage reference is somewhat arbitrary. 

So then my circuit does this.  There is a 25W Heater (current source) feeding your Al plate.  (I'm ignoring the thermal resistance in the source.. more below.)  This goes into the Heat capacity of your plate. (if you tell me the volume of your plate then the heat capacity is about 3 J/K per cm^3.)  This plate is connected to the outside world through a resistor to 300K.  So you can now go ahead and solve this.  And find out not only the final temperature of the plate, but also the time constant.. how long does it take to get there. 

OK now the resistance of the heating element.  You could add this in as a resistor between the current source and plate C.  You only care about this if you want to know how hot the resistor inside will get.  I assume the number in your link is to the case.  I'd further ignore the convection loss of the heater body, (assuming it's small in area compared to the plate.)  

Once I make this type of model I'll get a little thermo-couple and measure temperatures here and there and see how good the model is.  (It looks like your plate will get hot.)