The SC1000 design is reasonably straight-forward, and building something similar would provide you with a robust and reliable LF transmitter. A single-module 500W design would be the simplest to build, and I recommend that you stay close to the modular concept used in the SC1000. You should preferably build a Power Supply, Modulator, PA and LP Filter module, along with an SWR circuit. I'd recommend a 3U rack cabinet or other similar sized box. The modules could be laid flat and wired point-to-point, so no mother board would be required. The very simplest transmitter would consist of a tapped 50 - 70V transformer and an 18V transformer, a PA module, LP Filter and SWR circuit.
Start with a 115V output 500VA transformer. Use a decent sized relay with 24V DC coil to switch power in the primary side. Use a small 18V 1A transformer (permanently powered) for the LV supply. Use a decent high powered line filter, a 5A line fuse and proper mains earthing procedure. Replicate the Power Supply Circuit. Don't bother with the DC connections. A 7812 linear regulator with a good sized heatsink (maybe the chassis) should be OK for the 12V regulator. Use screw terminals for inputs and outputs. The required inputs are HVAC (two), LVAC (two); while outputs are HV OUT, 24V OUT, 12V OUT and several GND connections.Note: if you don't plan to use the switching modulator, I suggest you add a 500VA variable transformer (Variac™) in the primary of the HV transformer, after the fuse and relay (Don't wind it above 130V DC output!). An NTC thermistor in series with HV transformer secondary wouldn't be such a bad idea.
Replicate the Regulator / Modulator Circuit. Wire the SHUTDOWN input to a pullup resistor to +12V and a switch to ground. This can act as a TRANSMIT switch. Wire an LED and resistor to the switch as well, to indicate when the transmitter is on.Connect the HV and +12V supplies to the unit, and the modulated output VOUT to the PA. I'd suggest a front panel 10k pot in series with 47k to +12V as a power level control, to adjust the VMOD output. You might experiment with AM, by applying AC coupling 1V RMS audio to this point, with the pot set about half way.
Use a reasonably sized finned heatsink for the power devices (use TO-220 equivalents), and fan cool it. The fan is an overkill, but you neven know when the antenna might fall down while the transmitter is operating unattended!
What a simple way to achieve 500W on LF! Replicate the Class D PA Circuit. For drive, use a ZL1BPU LF Exciter, or other reliable and stable source with 0dBm output or more into 50Ω. Use really well shielded coax from the back panel to the CAR drive input. Use screw terminals to provide the modulated power VMOD and GND from the modulator. I suggest using separate 1mA meters to monitor the PA voltage VSM and current ISM. With 1mA meters the ranges are about 160V and 10A full scale with the given values and typical 1mA 100Ω meters.What to use for ferrites? I'd suggest two 35mm 3H1 toroidal cores stacked for T2, and a 25mm toroidal core for T1. Note that T2 is a transmission line transformer (1:1 balun), so has essentially no flux in the core. Each winding on T2 has about 10 turns of Litz wire, and on T1 (at a guess) 20 turns per winding. Watch the insulation on T1 secondaries carefuly, as they are 100V apart.
Use a reasonably sized finned heatsink for the power devices (use TO-220 equivalents), and use the fan to cool it. You might add a thermal switch to the fan. 12V fans are easier to find, so use two in series off the unregulated 24V supply to keep 12V current to a minimum.
Build a 5-pole Butterworth design with a corner frequency about 1.5 times the intended operating frequency. You can scale the values used in the original design, for example to change from 500kHz (original) to 250kHz (for Amateur use) double all capacitors and inductor values. Use air-cored inductors, as ferrite will give trouble at high flux densities.The LP Filter output at 50Ω goes to the antenna via the SWR sensing circuit. Outdoors, right under the antenna, use a large transmission line balun to match the antenna and a huge tapped series coil to resonate it. The series coil could be two plastic paint buckets arranged so one sits inside the other for adjustable mutual coupling. Use heavy gauge insulated wire for the coil - remember there's about 5kV across the coil and on the antenna! With luck and a good ground system, you might get 3A or more of RF into the antenna.