I just picked up a 89 Sidekick for my wife & it's got manual steering. I'm wanting to convert to power steering but what resurch I did told me I need parts from a 89 - 91. Will the parts from newer Sidekick work?

The 8 valve and 16 valve have different engine mounts, and is a pain to adapt therm.  The steering box appears to be the same and will cross over without any issues.  The intermediate steering shaft is a different length for power/manual steering, so get that also. 
It does make a difference, especially with bigger tires.

My biggest concern really is the steering box, I don't know why all I found was the 89 - 91 steering boxes are interchangeable. You would think the newer Sidekicks would be too.

Check the mounting holes to see if they are the same.  If it fits you are in luck, but the engine mounting brackets on 8v and 16v motors are different.  Just spoil yourself get the right motor bracket.
I change three of them around, but they just all happened to be same year.  Motor mount and steering shaft were the only differences.

My biggest concern really is the steering box, I don't know why all I found was the 89 - 91 steering boxes are interchangeable. You would think the newer Sidekicks would be too.

I don't recall there being any difference in the later steering boxes.  I know they changed the steering shaft (the early ones used a solid round type shaft and the later ones had an "accordian" looking section in them), but the PS shafts should all be the same length.
So, as said....PS box with arm, PS pump and bracket (from an 8V trackick), lines, reservoir, and steering shaft and it should be a bolt on deal.

will the brackets off of a 16valve w/power steering only, go on a 16valve tracker with only a/c?? or do i need to find brackets on a 16valve that allready has a/c and power steering to make it work??

I haven't seen PS w/o AC, but have had AC w/o PS.

guess ill just have to bolt on the ps and see if the ac has a way of bolting on, if not ill just fab something up.  thanks