Sounds like you've done some homework. The thing that you might not have had the "lightbulb" moment about yet is that there is usually not a magic system out there that does everything for every house and every homeowner.
Best thing to do when starting on any path to improve your comfort in your home is to improve the home itself. Anything that you do to reduce the infiltration of outdoor humidity, air and heat in and out of your home will also reduce heating and cooling costs, a win win situation. If you put off doing these things until after you've installed HVAC equipment the chance is almost 100% that the equipment will be to big and won't provide the comfort you were looking for and paid for.
The two stage air conditioning outdoor units on the market matched with a variable speed air handler/furnace being controled by a good control/thermostat will give you the best there is available out there to do what you want with your central HVAC system.
I understand that you don't want to spend money on both a dehumidifier and a new HVAC system. Can it be guaranteed that you will not have to? I'd have to answer that No. Again it comes down to the home, how you live it, the day to day actual conditions outside and inside your home and many other variables.
Your top floor will always cool less than the lower levels unless the ducting system was installed properly in the beginning. Sometimes this can be solved with modifications that are to numerous to list and are best left to someone that actually visits your home and sees first hand what you have and can best figure out a solution that you can agree to.
Good luck with your quest to resolve your issues. I'm sure others here will also have plenty to say on the subject.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.