i went with the sigma 530 dg super as my first flash, then i went with the metz 50 as my second. As much as i like the metz i feel another sigma would have been better.
Why? same menu system, the ability to work as a dumb wireless flash (triggered by flash light) so you can go full manual with your flash without the need of a radio or optical trigger (the metz was meant to have this but doesn't in the pentax version).
The metz is good, it has the ability to be updated by usb and has the ability to turn the auto power off (my reason for getting it). The last thing you want is your flash turning off on you when you have a little quite time and its up high somewhere on a stand.
If you didn't need hss or auto flash and wanted to go full manual there are cheep flash's on ebay with similar power outputs but are manual only. Some cheep radio triggers. A setup like that would cost around £250? for 4 flash's and the triggers.
Ive only use manual flash's so far off camera and didn't find it that hard when doing studio stuff, a few test shots and all was ok.. but when i was using my d3 with my sigma flash (had no nikon flash) being full manual on the body was a right pain when shooting at an event as i had to constantly change aperture or flash power depending on subject distance (a reason to get a p-ttl flash so it works itself).
Im still getting used to working with the p-ttl off camera with more than one flash so ill let you know how that goes on for me