donkey,
First, to be clear, you do mean 500mm x 500mm, not 500mm^2 ?
The key to even heating in both your table and cooking is to increase the thermal mass between your heating element and your work/food. Consider this: if you heat one spot on a piece of aluminum foil, do the edges get warm? The answer is generally "no". If you take a 500mm x 500mm x 25mm piece of plate aluminum and heat even one corner the entire plate will enter a thermal cycle where an equilibrium temperature above ambient is reached for the entire plate. In better quality cooking utensils the bottoms are "layered" or, more correctly, "laminated". The Lamination process usually involves different metals, typically aluminum, copper and steel. If you were to use 4 pieces of 5mm aluminum plate with 3 pieces of 1mm copper sheet between them the resulting thermal mass would heat considerably more evenly than the single 25mm aluminum plate, even if the heat source was not ideally distributed.
I do not remember if Oz uses 220V mains or 120V/240V mains, but in either case the use of a range element is dramatically easier than DIY nichrome wire. In addition to my electronics hobby, I am a home brewer. I began heating my boil kettle with propane, but have migrated to electric. I built a "stove top" for my boil kettle using 4 * 6in 1800w range elements for my 21in diameter boil kettle. To ensure even heating I use this pot:
http://www.ebay.com/itm/Precise-Hea.../251122819149?pt=Cookware&hash=item3a7816204d . Sadly there is not going to be an "off-the-shelf" heat bed readily available for you; however, if you will laminate alternating plates of aluminum and sheets of copper you should be able to fashion a heat bed that provides very even heating. I would think for your purposes a single 8in range element would work very well.
If cost/complication is not an obstacle, and you want truly even heating, you could make your bed from two pieces of aluminum plate with copper tube sandwiched between them. In this scenario you would heat water or oil in a flow through heater like this:
http://www.ebay.com/itm/Spa-Heater-...t=LH_DefaultDomain_0&var=&hash=item19d34e2d90 and use a pump to circulate the liquid through the copper pipe. As long as you don't need a bed temperature greater than ~90C you can use water, a much simpler/less messy solution than oil.
All of my suggestions have been >> 120W but use mains power rather than a low voltage supply. In your OP you stated you wanted: 1) Even Heating 2) Adjustable Temps 3) Super Quick warm-up time. 4) Low Amperage. I cannot see a way to achieve all of these goals from a low voltage supply. Ultimately (as pointed out above) P = IE. Since even heating is a function of thermal mass & thermal conductivity to achieve any particular temperature different than ambient requires a fixed amount of power + thermal losses over the time period in which the thermal mass is heated, AND because you wanted "super quick warm-up time" with low amperage, I cannot see a different solution than using mains Voltages. As heating and mains voltages are inherently dangerous, using existing, mature, mass produced, engineered products seems like the best course to follow.
The only two obstacles I see are 1) cost of the bed 2) Skill Set // Access to the tools required to make the bed. Aluminum plate is not terribly expensive, but copper sheet is a bit pricey. I would suggest looking for copper sheet in the form of "flashing" made for the roofing/building industry; while still driven by the price of copper, it is typically cheaper from building suppliers than other sources. As far as Skill Set // Access to tools, I assume from the nature of your project that you have both. Copper and aluminum can be cut/drilled using common wood working tools.
As far as inductive heating goes, I love the concept, but that is a pretty complicated path to start down. I have investigated inductive heating casually, but I have never been able to get the cost/reward ratio to align properly. Perhaps in another decade we will have switches that make it more cost effective, but for now there are just too many compromises for too little reward at too high a cost. It is a great choice in highly specific industrial situations, but the range of effective application is still pretty narrow.
Good Luck!
Fish
A lot depends on budget, space and configuration of the machine. Most of the 3D printers I have seen have a stationary bed which can make the task easier. While not "cheap", aluminum plate is a GREAT conductor of heat. While not particularly "fancy", a common kitchen range element is dirt cheap and easy to work with. Put the two together with a few DS18B20s, a SSR and a uController and you should have a pretty amazing bed. NOTE: most range elements are designed for 240Vac; however, they will work just fine with 120Vac @ less than half of their design power (still a gracious plenty for your purposes.)
