Hi, welcome at Amiga.org,
here are some more informations:
1. The monitor you describe seems to be a 1084S or 1085S (9 pin plug). The older monitors like the 1081 or 1084 have a scart-connector (like a TV).
Try to get a cable via eBay (or if you have experience in soldering and the right connector, change your existing cable).
2. If you want to build an Amiga that is somehow usefull (and not just for playing the old games) you should look out for Kickstart 3.1. Maybe you should try to get a complete OS 3.1 set incl. the manuals. The OS 3.1 manuals are very good and informative.
A Kickstart Rom could be gained via Vesalia (if you are from Germany or central europe):
http://www.vesalia.de/The default Workbenchscreen is still grey, but you could change the colors to your own taste.
3. If your Amiga has only 512KB Chip-ram, there is an easy hack to change to 1MB chip using a newer Agnus. I'll search for the link if you are interested.
To add fast-ram (upto 8 MB) you will need a ram-expansion card (or maybe you own a SCSI-controller that supports additional ram). Usually you have to add some kind of single ram-chips (e.g. ZIP-ram) to these cards. These chips are very hard to find, try ebay.
5. I think your Supra SCSI HD for the A500 could be shut-off with a switch, then you could see the boot-logo.
6. The Scanlock seems to be usefull, if you want to connect a video-recorder to your Amiga. The monitor should be connected direct to the Amigas monitor-port. You should try to find the correct name of the Scanlock and search here for a description:
http://amiga.resource.cx/General: Have a look into the Aminet, here you find thousands of programms, tools, hacks, and documentation for the Amiga:
http://ftp.uni-paderborn.de/aminetAnd you have to open your case to have a closer look to the parts (boards) placed in the Amiga.
AND AS ALREADY SAID: CHECK THE BATTERY!!!! If it is leaking, cut it off. You could replace it later, the Amiga will run without it, you just have to set the clock everytime you power-on the machine.
Noster