The Scart can be connected to anything "Scarty", the quality is better than of the 520 but not as good as with a regular monitor. As said already an external scandoubler might be what you need and then you should be able to connect your Amiga upp to a flatscreen monitor to save space and weight.
Amigas are as close to plug&play that you can get... apart from installing drivers the hardware will most often do all the needed confing for you. Diskdrives doesn't need drivers so you can easily just plug on an externa drive onto the A500 and instantly have 2 drives (you can go up to 3 external drives by connecting a second drive to the first externa drive like a daisy chain).
About keyboard I don't think there's a converter for PC keyboards for the A500, I might be wrong but I had a quick look around and didn't find nothing. An Amiga keyboard like the ones used in an A3-4000 is slightly slimmer than the A2000 ones (the CDTV keyboard is as the A4000 one but all black and extremly hard tyo find). If you are upp for customificasion why not try and work something out with an A600 keyboard, they are slimmer but I don't know much about them (apart drom the fact that they got a smaller green keyboard cable than the other Amigas doo).
