if u ask me...
get an accelerator card (030 with minimum 16 mb ram , but theese days thanks to HAYMIGGA (here on amiga.org) you get 128mb simms for VERY little money)
next get OS3.9 and 3.1 roms (hardware).
and if you want to use a cheap 3.5 Hdd then you can do that by getting an EIDE99 or simmilar , but the prob here is, the hdds are too big for the a1200 case so normally u would have to put the hdd ourside of the a1200 (and that looks abit messy)..
also.. an NOrmal PC PSU (atx or at) is something you should consider aswell (some minor cabling is needed)..
anyway.. quick ref:
1. Os3.9 (operative system...)
2. 3,1 Roms (hardware roms to update hw to work with os3.9)
3. Hdd (either 2.5 (laptop pc style) or 3.5 (normal)
4. Cdrom
5. PSU
6. Accelerator..
none of theese above is REALLY needed but i recomend them to you as it will make your experience so much cooler..
cheers