Quality wise, the 4500's mechanical design is way above the 9000.
The 4500 is at its end-of-life for the electrolytic capacitors, which means you'll need to have them all replaced, if you want to enjoy continuing trouble-free operation, with the best sound possible.
Just because the stereo "works", doesn't mean that all is OK.
Martin Dillen sells capacitor kits.
The BC9000's CD door issue is also a capacitor-related issue. Do the fix (and it is a simple fix - replace C2103 on the servo board of the BC9500 - it may be the same on the 9000), and it's good for another 7-10 years. In about another 5 years anyway, you'll need to replace ALL the electrolytic capacitors in the 9000 as well.
Either make sure you're comfortable with this kind of work, or find a good electronician who will do the exact work you specify, and appreciates the difference between "a quick-fix repair at maximum profit", and "a full-restoration" so you don't have to visit him again for 10 years.
Learn from the mistakes of others - you'll not live long enough to make them all yourself!