Expect to see at least a 30% reduction in your power bill. Units now are much more efficient than 20 years ago. Keep your coils clean and change the filters frequently and you should be good for another 20 years. Good luck!
Just get a couple of starter capacitors for the condenser unit. My system is 6 years old and the starter cap went out, I could get the system running by manually starting the fan spinning with a screwdriver, so at least we had some AC. The AC tech was saying that the new caps only last 4-5 years, so we had a good run, but I call BS on that. Those caps should last the lifetime of the system, but then again nothing is built to last these days.
I went ahead and bought a new cap and will have it on stand by if it ever goes out again.
Been thinking about adding a mini-split to my office. It's the "bonus" room above the garage and is hot in the summer and cold in the winter, pretty sure the insulation is also sub-par but don't feel like pulling off the sheetrock to replace it.