R – Need technology recommendation/suggestion

My company is in need of a task management system to handle scenarios as simple as "Purchase a computer for X" to "Relocate a person to another country". The simple scenarios are a single tasks handled by a single person, whereas bigger tasks can be broken down into multiple sub tasks delegated to multiple people during the workflow. Additionally the clients and vendors need their own views into the process.

We are evaluating different solutions from a custom application built on Workflow Foundation to SharePoint to BPM products like Metastorm and BPM.Net.

Here's my current understanding of these solutions:

1. Workflow Foundation – Low level workflow designer and/or library with no host environment. It seems we would have to reinvent some wheels if we went this route such as fault tolerance and document management. Some of the answers on stack also cause concerns such as the lack of versioning and a complete overhaul for VS10/.NET 4.0
2. SharePoint – Built for document management and collaboration but trying to create advanced workflows and tasking on top of that seems like a hack. Plus all workflows have to be tied to either documents or lists. I cant envision how a list (or list of lists) can address this issue.
3. BPM products – Mature workflow engine at a seemingly high price. BPM.Net is the only solution for which I could find some level of technical detail but im still not sure how different developing against this product would be from developing against Workflow Foundation.

Are there any workflow engines dedicated to solving all the workflow pains that can be easily deployed with their own hosting environment and initiated through a webservice?

Are there any other options I am missing?

Thanks in advance.

****Edit**

To answer the questions below the workflow needs are pretty light. Basic routing of tasks to approvers and subcontractors.
Whats driving us too look deeper than PM software is the nature of the business not the need for advanced workflow. We are basically in the business of procuring goods and services through subcontractors for our clients which can also include full employee relocation. The interface of the package should reflect this by being customer branded as well as intuitive for this line of business.

Basically if im moving my family to the other side of the world Im not sure i'd want to interface with Jira or Sharepoint or any other PM software to facilitate this.