The A600 is a very nice machine.
Surface mounted chips may be a hassle to replace but it is possible to do so if you have a good soldering iron, magnifying glass, a solder remover and a very steady hand.
As for expandability; I think they are hopeless! Sure you could add PCMCIA RAM, and/or an accelerator. But it's easier finding a dodo egg.
The harddisk is nice but most whdload games need fastram. Which is a problem as stated above.
The network card possibility is nice and gives it an edge. You can make a perfect ADF writer from it.
The footprint is nice. The ugly A500 slab takes up a huge ammounth of space. One thing...A500 tend to be less sensative to yellowing.
I'd go (and went) for an A600. Also because theyre cheap and easy to come by here in good old Holland
