New to A500/C64

[KThunder]

    you can also use adide or adscsi devices from icd or similar devices these fit in the 68000 socket and take the 68000 on top.
    there are also schematics and drivers that you can find online that you can use to build one of these devices. they are actually quite simple. all of those are nonbooting though, so you would have to use a floppy to boot and load drivers.

[KThunder]

1. max size for the hd would be 2gigs unless you do a bunch of upgrades (68020 accelerator, 2megs ram os 3.1 kickstart, and cdrom) and used os 3.5 or 3.9
2. trapdoor can take specially made 512k cards several boards are made that attach to the side and allow hds and other types of ram
3. uae- unix amiga emulator works really well i use fellow for dos alot for games
4. null modem cable i think there is a parallel port setup you can use too, check uae docs
5. buy amiga forever from cloanto  it has all the emulators, all the roms, all the os files and great documentation as well as some software including personal paint.  

[KThunder]

you can use 6.4 gig hds but only 2 gigs will be usable
more ram is good but old games and many apps only need 512k to 1-2 megs at most
yes os3.1 rom is a very good thing