A SWCG should not change your pressure by more than 2 PSI but depending on the heater and how it is plumbed can add quite a bit. If you put in a flow bypass with an adjustable ball valve you can set the flow through the heater to be just enough to activate the internal flow switch while bypassing most of the water.

If you take your lowest pressure today which is 18 PSI, you could be running with head as low as 55 feet. This is pretty close to 74 GPM which is the limit of your filter today. Given that you are so close to your filter limit, I would say that a 1.5 HP will probably push you over your filter limit. Without more accurate head measurements, it would be risky.