Hi Jeremy, I picked up the heatsinks from a scrapyard complete with all the transistors, for a few dollar each.
Yes the current is adjustable, in operation it acts like a bit "adjustable resistor" with a pot and an on/off switch. (Don't forget the on/off switch, it should switch before the gain stage, the last thing you need is to be trying to switch heaps of amps on/off or pulling connectors to break the current!)
As for build costs and max watts, it all comes down to the cost of your main dissipator. I used the power transistors as the dissipator as this makes it useful for many different PSU voltages, ie can use it on 5v, 12v, 36v etc supplies. If always working with 24v PSUs you could do 75% of the dissipation with a cheap resistive load (like a jug element or huge handmade wire wound resistor?) and use the transistors/FETs to control the current through that resistive load.
With your 8 FETs on the water pipe and you said you want 1700 watts I'm worried about reliably dissipating 212W per FET! What package are these? My dummy load uses huge TO-3 aluminium body transistors which are specced at about 115W max dissipation each. Obviously I allowed for a large safety overhead.
If you are using TO-220 FETs even if you cool the tags with water cooled copper pipe the TO-220 pack thermal resistance (chip to tag) still only allows a safe 50-60W dissipation per FET. Or did i misunderstand and your are using some massive industrial package FETs?