1. Should we use the term Holy Spirit or Holy Ghost? Or does it make a difference
In modern English, "Spirit" conveys the meaning better. In modern usage, "ghost" is usually used to refer to someone who has died that haunts a place (a false belief) and may cause unbelievers great confusion.
2. When we get to Heaven - will we be able to see the Holy Spirit?
Probably. The question seems to assume that "spirits" are invisible. I know of no reason to make that assumption.
3. We say that the Holy Spirit is the 3rd Person - how is he a person without a body?
Unless one is a Mormon, this shouldn't be a concern. We could ask the same thing about the Father -- How is the Father a person without a body?
4. Why does God the Father need the Holy Spirit to fulfill his will?
I don't know of any biblical passage that makes this assertion. The Spirit has eternally existed, with the Father and Son, in an eternal union of the Godhead. If the Father is doing something, the Son and Spirit will be involved. If the Son is doing something, the Father and the Spirit will be involved. If the Spirit is doing something, the Father and the Son will be involved.
5. What Roll did the Holy Spirit have in the OT?
The Spirit had an extensive role, from the first moments of creation ("hovering over the face of the waters") throughout every facet of God's work. Often it was not specifically called out, but one can see the work of the Spirit much clearer and more explicitly than the Son's work in the Old Testament scriptures. The Spirit even gifted the craftsmen constructing Solomon's Temple.
6. What roll will the Holy Spirit have in the tribulation
(Note: this is NOT a discussion about the tribulation -)
A question about what happens in the tribulation is certainly about the tribulation, despite your assertion.
We have been in the tribulation since the Spirit was given without measure to the people of God. The Spirit convicts us of sin, righteousness, and judgment. The Spirit empowers us with giftings and guidance, aiding our communication with the Father and Son by interpreting the deepest groanings of our hearts. The Spirit is with each disciple individually and corporately as we go about our business upon the earth, and will remain with us in the resurrection and through all eternity.