Of course you have to worry about the fact that you are heating a portion of metal to be hot to the touch/burning... For 1 minute per level max or concentration of the caster.
For this to work you need two separate casters, and both must be able to produce water and have iron. You will be able to power your device for... 2-4 minutes/day.
Meanwhile you are paying for ten laborers worth for the casters alone, and then needing to pay 10 GP per casting/day. With that kind of money I can buy 400+ laborers to do my dirty work for me... Why would I hire your machine specifically?
First... how are you figuring out that amount of Joules? Your agitation can affect an item with surface area of 2 square feet... Overall a roughly 4.8*4.8" cube on each side can be brought to scorching temperature... And you have to figure out how much energy is being produced by 1d4 damage, then
translating that into Joules, then into an actual Horsepower/hrs run... And then translate that amount of money into the worth of purchasing a pack animal over a year's worth of spellcasting cost.
A merchant presented with a requirement (needs 2 casters on payroll + costs of services)... or investing that rough cost (2 1st level casters wages and spells will run 15320/year) to purchasing pack animals/wages.
405 laborer's wages... 1825 mules... or about a day's worth of work from a steam-powered weirdo.
The costs of the spellcraft you are discussing != profitable in many cases. There are uses for your abilities... Short-term bursts, trading. Personally I'd think that a 1st level caster being able to provide a nice platform to lift 100 lbs along with their own carriage up to deliver supplies to a group of workers would be much more useful. And even then it isn't THAT useful to pay a half days work for it.
But do you know what is useful on a construction site?
Prestidigitation: Great for the 'end of day' tidying up of the space. Of course 1 hr of work is going to not be worth the same as just having five apprentices cleaning up the site Heating up and flavoring food brought from home is a nice touch on the site, and you can also clean up the worker's clothing as they collect their wage for the day (and have a happy laborer going home without that whole 'bathing' thing

. Grade: C.
Feather Fall: A literal lifesaver on a job site. A commoner who fails a Climb check is completely screwed if they fall. Having a construction Apprentice of 1st level about can save lives. If you have an Int above 12 you can save 2 men from falling (though not at the same time). Or...
Silent Image: The best use of a Construction Apprentice. Draw up the plans... If he can ken the things (Knowledge [Architecture] check)... And now you have a three-dimensional depiction of the building to be worked on. The image can be marked up by the Mage for the day for specific sites, and (in theory) with names of who is working what area.. And all he has to do is sit on the site in a nice quiet spot within view of the location (440 ft minimum).
Now a 3rd level caster would be much more useful as he can carry his own carriage +300 lbs up and down a staircase (maintaining the 'ground below' rule of the spell). A full day's work with a relatively strong man can help to carry materials that would be hazardous to carry (boiling tar in a cask, for example) and save laborers dying.
So if I was doing a very, very nice project I would have apprentice casters there to Feather Fall, and 3rd levels or higher to use the Disk... And keep a Silent Image up and running to depict the system. They can be useful... But not gamebreaking.
Slainte,
-Loonook.