As for item 1, any audible differences, if any, will be minimal. And whether you will be able to hear them is also a personal thing that only you can answer. I would suggest you "rent" one and see (hear) for yourself if you can hear any difference.
Now, as for the functional features your current CD player lacks, once you do the listening test, you will have to decide whether it's worth the difference to you.
If you choose to not keep the Marantz, you can always return it from whence it came for a minor restocking and shipping fee. It's worth it so you can hear for yourself, isn't it? After all, we don't have your ears.
As for your second issue: How much power are you looking for? How many channels are you looking for? How much are you looking to spend?
You do know that for a barely audible 3 decibels of loudness in peak situations you need twice the power you have now, don't you?
Since your speakers are incredibly efficient to begin with, you might seriously want to rethink the practicality of this one.