I cannot think of any single suit that would do the trick from 70F down to 25F or so. Every "insulated" combo I know of does not have a way to remove the insulation from the bottoms, though some have a combo coat/jacket set up. So I would suggest a relatively light weight soft, "silent" outer shell bib and coat that is both water proof and can serve as a decent wind breaker. Then layer up under it with high quality LJ's. I have such a set-up that does me fine to about 20F. Then for really cold ... or cold and very windy weather, I ahve a typical water proof insulated bib and coat combo.
The uninsulated bib/coat combo gets a ton of use. I bought it through Cabela's. I think it was marketed as "Silent Hide" or something like that. Several folks market such a combo ... Columbia, 10X , Red Head , etc. Mine will not will not hold out driving, heavy rain all that well. I get wet around the shulders and upper leg. I seldom hunt in this sort of weather so this is no big deal. But a typical rain shower, sleet or snow it does fine. If you decide to layer up with LJ's, if you did not know already, stay away from cotton. In cold weather, cotton can be really bad news. It holds moisture and therefore if wet will draw off body heat like an ice bath! Stay with a product of wool-poly blends, or such. These are a bit pricey, running about $50 and up for a top & bottom, but should last for years if taken care of propely, and do a fine job of keeping a person warm with relatively light weight.