Question 1)I need to know if this is straight forward ?
Yes.
Questions 2) is a there an upgrade version path to follow ? and what are the specific reason ?
The recommended upgrade path is to install the latest 6.5.x, and then upgrade to 8.5. This is recommended because it is tested and supported by IBM. The client in 5.x used different install technology which is accounted for in the 6.x (and 7.x) installer but may no longer be working in the 8.5 installer since it hasn’t been tested.
Question 3) If there is a path to follow must the Notes Client be launched to the upgrade at each version install ?
The ODS version has changed since 5.x at least once. While the Notes client is always backward compatible (Notes 8 can read all previous databases no matter what ODS level), the tested and supported upgrade always assumes the user had run Notes at least once prior to upgrading again.
Subject: IF you want to do it officially right, you have to go to 6.5 and afterwards to 8
You should do the complete steps to 6.5 and run the system for a few weeks before going on
(Yes, the client would have to be startet (e.g. conversions of bookmarks and names.nsf).
On the other hand I did upgrade a 3 user Installation from 5 to 8 without any problems - but this users did not even have a lot of folders, low budget, I communicated the short cut with a lot of risk, everything was still there to do it once more slowly, if needed etc etc.