I'm guessing you might have already done this, but just in case you haven't. When we were planning on changing planes a few years ago, we did like you are doing on requirements (one list), then types we liked that would fit the requirements (second list), budget (most important, but shortest list!) and sat down to do a comparison.
The C180 seemed to fit the bill. We spent $70k initially for a 1955 model, and have done a number of upgrades at the last two annuals, including a Garmin 430 so it is fully IFR capable now. We view this as a longterm investment, so worth putting the money into. It doesn't do the 600nm range that you want, but would if we had changed the fuel tanks to the larger ones. We have just completed a return trip from Oshkosh, WI to Santa Monica, CA. 25 hours of flying time over a week, with comfortable 2.5 - 3 hr legs. The average TAS is about 150kts at 7500 feet, and fuel burn is about 11.5 - 12 gph. Getting up high to get over the mountains was not an issue. It is so stable and flies so straight, no autopilot is necessary, IFR or VFR. Lots of space for luggage, and we could have filled it with more people easily.
The hot rod Glasairs and Lancairs certainly will go a lot faster for a better fuel burn, but the 180 will take us on cross country flights or mountain flying in Idaho on rough strips or we can load it up and go camping in the summer. It is a good multi-tasker and for us, it is the ideal aeroplane.