That sounds like a great idea for a themed necron army. I can answer all of your questions now, but in two weeks there is going to be another necron codex released, so the answers may change.
1) Yes, max is 5 in a squad, but 3 of them can be upgraded to heavy destroyers.
2) Immortals cannot be transported in a ghost ark, but can be transported in a night scythe (15 of them) or be placed by a monolith during the game. (any sized unit)
3) Triarch Praetorians are jump infantry.
4) Im not really sure what you mean by the last question, but I can tell you destroyers/heavy destroyers are very good from long range, and praetorians can be good too.