Welcome back!
What do I need to add a bigger HD ~10GB
You could best use SmartFileSystem (SFS in short) - available on Aminet. You can also find a link in the newsitem about SFS on the frontpage of this site (as long as it's there at least).
I'm still trying to figure out SFS myself (I'm not much of a technical miracle so I still don't understand what I'm doing :-D ) but it's the best way to use harddisk larger than 4 GB (and it's free).
What do I need to add a CD-rom
Depends on what you want. You can put an internal slim CD drive in your A1200 case, using the internal IDE controller. You can also buy a Squirrel SCSI controller, or a turboboard (e.g. Blizzard 1230 with SCSI controller), and add an external drive. You can also find yourself a nice towercase in which you can put pretty much any CD-Rom drive that exists. The possibilities are endless :-)
What do I need to add a standard PC monitor
You will need at least a scandoubler, and preferably a flickerfixer so you can display interlaced screenmodes without any hassle.
I suggest you should take a look at the photoalbum on this site - look for the Hardware Amiga 1200 section - it will give you plenty of ideas what other users have done to their machines and it may give you an idea on what you want. It helped me a lot with my cooling mods for instance...
Cheers,
Eric