2) 3 turns (or rounds).Is there a maximum distance to trade?
Yes. Your "exporting" colony must be within 24 squares of the "importing" colony(ies).
To check to see how many squares apart they are, click the "show grid" icon in the upper-right corner of the main map screen (click here for an example).
3) I don' t know. Where can you see what you need or what our ally needs? is it visible in the "table Review trade/Set Embargos" (screen diplomacy)?
It's not what your neighbor needs, it's what your colonies have to export (whether trade goods and/or tourism).
To see if a colony has something to export, click a colony, and then look in the upper-middle spot (again, see examples here and here).
3a)I have a Commercial Spaceport in my colony....but my ally has no business space port or Landing Station
So long as you have a trade mission built on your ally's colony, you should be fine.
However, to trade with your *own* colonies, you will need Landing Stations built on them.
"Evil is easy, and has infinite forms." -- Pascal