As others have pointed out, buying a used or partially completed homebuilt is almost always significantly cheaper than starting from scratch.
A new IFR capable 4 seater will almost certainly cost north of $100K to build, and there are few out there. Used, probably not much different as there are so few out there. Certified, likely cheaper as there are a lot of older 172s, Cherokees, and the like out there, and some (but by no means all!) are IFR. And there will be many times when even an instrument rating won’t get you through bad weather if there’s convective activity or it’s below minimums… or even above minimums; if you don’t fly IFR regularly than your personal minimums may (should be!) higher than the legal minimums.
A 2 seat fun plane combined with renting or clubbing a 4 seat traveling machine works for some people, but availability can be an issue if several people want the plane for the weekend. Some clubs are great, with a bunch of pilots who rarely fly, but others are more problematic.