Have you considered a Heat Pump?
With winter extremes so mild, I'm have to assume at your summer extremes are high. Are you in a humid climate (A.K.A. green grass)? Either way sizing for your cooling load may drive the equation for the air handling system. However without qualifying location no meaningful answers can be given.
That said, it might be to your decided advantage to consider a Heat Pump instead of a fossil fuel heating source. They are at their best in the temperature range you're indicating. Given the current fuel prices electrical may have a decided long-term advantage since it isn't exclusively dependent upon somewhat questionable sources and depleting national reserves.
Please Note: I’m not pro/con either way on equipment fuel type. In fact natural gas furnaces have a decidedly longer design life (tried, true and proven) than refrigeration equipment. It’s just that the realities of fossil fuel have changed over time and we all need to consider the finite nature of that resource.
Sometimes speaking with local contractors who deal with your issues on a professional basis has much merit. Ask each of them to explain their recommendations in terms of initial investment and operating cost verses comfort provided...