The fact that you've been heating with a couple of Rinnai space heaters means it can probably be heated reasonably with just two mini-splits. The output of the mini-split heads probably doesn't need to be as big as the output of the Rinnai heaters though. You could just install a couple with the same output of your space heaters, but if oversized they would be more expensive, and would cycle on/off rather than modulate, running at slightly lower efficiency, more noise, and greater swings in room temps.
It's possible to do 90% of a mini-split installation as a DIY, then have a qualified tech do the final pump-down, refrigerant fill, and system testing, keeping the whole thing under warranty. The hardware itself isn't outrageously expensive. A MSZ-FE18NA with brackets for mounting the exterior unit on the wall (above the snow line, protected from roof avalanches by the rake or eaves of the roof) plus refrigerant lines adds up to and puts out about 22,000BTU/hr @ +5F. The same unit installed by a high-end installer who backs it up runs about $4400. The 1-ton version runs about $2K with the associated hardware, and puts out about 15,000 BTU/hr @ +5F. The 3/4 ton would be under $2K, and puts out better than 10,000BTU/hr @ +5F. A 2-head single compressor 2-ton would run about $3500 for the all-in hardware costs.
It's worth reading up on how to install them before going the DIY route, but if you have it all set up reasonably it would probably cost less than $500 for the tech time to pump down charge & test two separate systems.
It's possible to do 90% of a mini-split installation as a DIY, then have a qualified tech do the final pump-down, refrigerant fill, and system testing, keeping the whole thing under warranty. The hardware itself isn't outrageously expensive. A MSZ-FE18NA with brackets for mounting the exterior unit on the wall (above the snow line, protected from roof avalanches by the rake or eaves of the roof) plus refrigerant lines adds up to and puts out about 22,000BTU/hr @ +5F. The same unit installed by a high-end installer who backs it up runs about $4400. The 1-ton version runs about $2K with the associated hardware, and puts out about 15,000 BTU/hr @ +5F. The 3/4 ton would be under $2K, and puts out better than 10,000BTU/hr @ +5F. A 2-head single compressor 2-ton would run about $3500 for the all-in hardware costs.
It's worth reading up on how to install them before going the DIY route, but if you have it all set up reasonably it would probably cost less than $500 for the tech time to pump down charge & test two separate systems.
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