#1 to #3 have roughly the same meaning; but #4 means "I was at the point of going" – in a context such as "I had got ready to go, and was preparing to leave the house, when X happened, which prevented me from going."
#5 isn't possible; "I were" can't stand as a main verb.
For the additional "modal" version, which modal verbs have you already considered?
All the best,
Student or Learner