I'll try : Posi trac is when both wheels put power to the ground .
non posi : would be where only one wheel puts power to the ground .
"How do I decide what to put in my car?" your choice .... what are you going to do with the car ?
"how do I know what's compatible with engine, trans etc?" Again what are you going to do with it ? what trans auto/manual? what you thinking of running under the hood ?

If your going to run 1/4 miles then build it for that ,Daily driver ? build it for that ....... So what direction you wanting to go ?
Tech gives a good explanation of gears some where in a thread but I couldn't find it . .... " I'll keep looking "

hope that gets ya started ....
T L