Perhaps my favorite statement!
I like the look of the WFM8300 posted by LethalWolfe, but make sure you're using the 3G input with the Kona3G to maintain your quality throughout; it makes no sense to have a 3G scope and output composite, for instance. The Tektronix model I use most often seems to be discontinued and besides the menu is a bit of a pain, but most of their products are superb.
Others have suggested various amps and speakers, which I don't have a strong opinion on anyway. I do like the Dorrough audio meters, though, so get the 1200 for stereo loudness and 5x 340-C, 1x 240-C (plus the rackmount!) for surround. Now that most of us[1] have to worry about dialnorm, get the Dolby LM100 as well.
Please use them appropriately, too; one of my clients has an editor who thinks the -20 indicator shows clipping and runs his nominal signal around -45 and a second guy who thinks -1db is a good target.
Video monitoring: I think the current revision of what we have is the Sony BVM-L231, but the way model lines go I'm not 100% sure. Make sure you get the appropriate option card to handle the 3G from your Kona. If you're doing client screenings in the edit room, you can be cheaper on the huge "wow" monitor but will want something like this with good color representation for your work.
1- yes, I realize you might not be in the same country as me, but your profile doesn't specify and most of the other recommendations are based off of a US-centric format anyway.
I have 3 IMACs with final cut studio. Now I would like to upgrade and have server setup etc and an awesome computer for editing.
small video editing studio with 3 IMACs.
Which is it -- do you want to upgrade and have a server and awesome computer, or do you want to use your existing iMacs?
so mid level budget will do. not so high and not so low also.
Again, which is it? Before I saw the "mid level budget" comment I spent 30 minutes researching model numbers (and others have put more time and effort than me) working under the "budget is not a problem" comment. What is "mid-level"? To me, that might be $30,000; for you it could be $2,000, for someone else maybe $75,000.
As gameface points out, you're really limiting your I/O and storage options with an Imac. It will constrict your ability to ingest, monitor, and output as well as dictate how you need to store your projects.