what MIGHT work and be simple (haha):
get one of those Bosch BOVA 3T or 5T ODUs, then put a coax/brazed plate exchanger indoors. they modulate from 25-110% based on load calculated by the ODU. all you'd have to do is control the water flow rate through the indoor hx to get the ODU to modulate up/down.
i've seen the 3T (0.75-3.6 ton range) available online for under $2k USD.
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