@kefter
The Overdrive Harddrive is worth at least £20 to collectors. The fact you have the box will help. Let's hope someone will want want it though, as the unit itself is not practical at all these days due to the use of internal CF drives or internal 2.5" laptop hard drives.
The Blizzard is definitely worth £130.
The software and disks boxed or not aren't going to fetch much I'm afraid, although AMOSPro boxed in good condition with all disks and manual is worth at least £10.
It's difficult to put a price tag for the items collectively. It will depend on the condition of the A1200 itself. A yellowed Amiga isn't as desirable as a white one. If the Amiga is perfectly white, then the whole lot could sell for £200.
@Darrin
I used to use an Archos Overdrive myself. Or was it Zappo? I can't remember, but it was the same device regardless of the name and mine had a Seagate 260MB 3.5" IDE HDD inside - Model ST3290A. Yes, standard drive which you could easily upgrade. They were marketed as such that you didn't have to break the warranty seal on the Amiga to fit the drive, and also, for those lacking technical expertise you didn't have to open the computer up either. They worked on the A600 as well as the A1200. Nice units actually. They actually boot from drive CC0: and then a tiny program on the carddisk (where CC0: was 30K in size IIRC) then mounts the hard disk and transfers control over to it, so it's then able to run the hard drive's own Startup-sequence. For instance, if you booted Workbench from floppy, then your hard disk partition(s) won't appear until you open up the CC0 drive disk icon and run the program in there called "AQ" (AmiQuest, and it had an AQ lettered icon).