Hey, welcome to Amiga.org :pint:
I'll try to answer some of your questions, others will fill in whatever I don't know, :-) . Whereabouts are you, btw :-? .
1. The A2000 video out is a 23 pin plug, which changes to a 9 pin plug on the monitor side. The plug that goes into the back of my 1084S only has 5 actual pins in the plug, but it is a 9 pin socket. There are also 23 pin to SCART cables around as well.
2. The 2.04 rom is the latest one that was actually shipped with an A2000. However, there are 3.1 roms available. (3.1 is the latest version). There are several shops around that sell them, try amigakit or Software Hut. AnythingAmiga in Australia also sells them. You can also 'Softkick' your A2000 to 3.1, but you'll need a legitimate ROM image. Workbench version goes with Rom revision - 1.3 can run WB 1.3, KS 2.04 can run WB 2.0 and 2.1 as well as 1.3, KS 3.0 can run 1.3, 2.0, 2.1 and 3.0, and 3.1 can run all of them up to 3.9. Sounds like your A500 was running 2.0.
3. A2000's have a max of 1mb Chip ram without the use of a device like DKB's Megachip. You can then add up to (I think)8mb fast by putting some on your A2091 SCSI card, or on RAM expansion boards. I've got 1 mb chip in mine, plus 5 mb fast on the A2091 and a Ram board. You can then add more if you use an accelerator board.
4. Well, there's a few 030 expansions you can find. The 2500 came with an 030 Accelerator card in it that ran at 30mhz instead of 7. GVP made them, as well as DKB and Commodore. Search eBay, A2000 accelerators turn up all the time. I'd say the GVP cards are probably more common on eBay. Throw one of them in, plus a graphics card, and you'll have a nice 3.9 ready 2000. You also might want to look at getting a 100mb SCSI Zip drive, they're easy to configure and a great way to back up stuff.
5. The Kickstart logo (White screen with hand) will only come up if you don't have a hard drive. If one is detected, it will immediately boot from that, so you won't see the screen. I don't know why you still saw it with your Supra SCSI HD though :-?
6. Don't know too much about the Scanlock thing. If you can get the monitor cable, I'd say bypass it completely, but you will get quite good quality with that if you use it to display on a TV. It's much better than using a 520 RF modulator.
Hope that's been some help. :-D
EDIT - Also, make sure you check out
The Big Book of Amiga Hardware. This should answer any questions you have about most hardware oriented stuff.