Searching monitoring tool for website process

Dear all,
I am looking for a web monitoring solution for our SaaS tool (www.decareto.de). One would assume there are lots of options out there, but I did not manage to find a product that fits our needs: We want to monitor uptime of a long running business process. I created a URL that triggers the process, which runs between 1 and 4 minutes, and displays a success message in the HTTP response.
Most of the tools out there do not allow response times of more than 60 seconds or so, and if they do, they do not allow to check for a success message in the response.
Any recommendations for a tool that might help us?
Thanks in advance
Eckhard