A couple of things on the IFS - First, The RSector and Exotek conversions have the IFS feature, so you can use either that, or the "stand up" shock version.
On the IFS, itself....the reasons are personal, but the IFS is harder to adjust right. It is heavily dependent on correct shock spring rate, and damping, to get right. The stand up shock set up is set and forget. I know it looks cool, but isn't a good choice for drift. Still, if you MUST have it, both carbon conversions support it.
Another reason I suggest a carbon conversion is, either of those conversions don't bottom-load the battery. The Exotek is a side-load, and the RSector is top-load. MUCH better and convenient arrangements.

It won't make any difference, having both a center and front one-way. They do not "complement" each other. They both have the same function, but give different results. Having both, however, won't hurt anything. I have a Tamiya TA05 VDF (the original first edition run), with a 1.60 CS kit installed. That came with a center one way pulley, and it is redundant, meaning it has no real purpose, since the front "diff" is already a one way.