I use one of the Micromys mouse adapters. Back when I bought the V3 I have, it was cheaper than other options. It will work on my C-128 when I want it to. I understand the V4 is even better, but the only real difference I see is that my V3 has DIP switches to set for the mode you want, and the V4 auto senses what you have plugged it into. I still use a wired mouse, just because I like them better. It is a laser USB mouse though. Works great, and no ball to pick up dust.
To cleaning up the look - retrobryt is a never ending task. Unfortunately its about the only option for the keys that makes good sense.
BUT for the case (and PSU if needed) I would just paint them. Use this finish - they have colors that match the Amiga quite well! It's made by Krylon, which can be bought almost any where. I have used this stuff, and I love it. The spray cans will give you an excellent finish too, if you are any good at spraying at all. Look for:
http://www.krylon.com/products/fusion-for-plastic/This will give you options to 'Mikes ideas. Good luck......
BTW - We used to be able to by a cable adapter that allowed using an ATX PSU with the A500. I can't seem to find one now. You could build your own, or get this and adapt your 500 plug onto it -
http://amigakit.leamancomputing.com/catalog/product_info.php?cPath=21_84&products_id=998
