Just in case the prior answers didn't cover everything, to summarize:

 

1.  Can I use my existing clothing and hair with the Genesis character?

Yes, although like any clothing converter some items convert better than others.  As mentioned, shoes are the trickiest.  Gen 4 conforming clothing and conforming hair can be converted to Genesis without anything extra; Gen 3 items require the purchase of the corresponding Gen 3 iconic shape to get the autofit clone for their clothes.  Note that clothes autofit to Genesis will work on any shape, not just the shape the clothing was original made for.

2.  What about character textures?  Will I have to repurchase my asian or african american textures?

Gen 4 textures can be used on Genesis without anything extra.  The default UVmap for Genesis is the V4 UVmap -- to use old M4 textures you need to change the UVmap in the Surface tab.

3.  I've divided my runtime into multiple folders (one for m4 clothing,  another for vehicles etc.)  Will this be a problem for genesis clothing items?

No, DS4.5 can use multiple content folders.  Clothing made for Genesis will be in DS format, while clothing for Gen 4 figures will be in Poser formats, but both kinds can be in external runtimes if you like.  Genesis morph sets need to be in the same content folder as the Genesis base, just like Gen 4 morph sets need to be installed to the same runtime as the corresponding base figure.

4.  Will I still be able to mix and match clothing like I can do with M4 and V4?

Yes, you can still mix and match.