I've deployed both K2 and WF systems in the past. K2 is quite strong, but spendy. WF is an underdog, but improving quickly. Both integrate with the .NET stack (MOSS specifically) quite well and both have very good tool integration. Both are relatively easy to develop for once you understand the workflow model.
You can get solutions support from many different MS partners for both, although my guess is WF is a bit easier to get solutions support for (i.e. more partners have more consultants who know WF than K2).
Unfortunately, I don't have any experience with the Oracle product or the open source alternatives you mentioned, so I can't comment on those.
If you are overwhelmed, I recommend you take a look at the WF Virtual Labs (bottom of the page). They will let you get your hands on the technology, get the lingo down, go through a few scenarios. Once you have that, understanding how WF can fit into what you trying to do should be substantially easier. Also, I can recommend Essential Windows Workflow -- very good book. Here's a good intro on WF 4.0 from PDC.
