1.)  Items modelled for a specific figure are generally meant to be used with said figure...they contain fit morphs for the figure and are generally modelled with their bodyshapes.
That said, you can use deformers to make changes and fit them to the figures...how?  I don't know, because I'm horrible with Deformers.

2.) Whether or not you postwork is up to you.  You can get sufficient tools with The Gimp, and it's free ;)  So, whether you spend the money on Photoshop, Painter, or PSP is up to you ;)

3.) The more items in your scene, the longer it'll take to open.  That's just simply memory usage. 

I have
AMD Athlon 64X2 Dual
Core Processor 4400+
2.21 GHz, 2.00 GB RAM
GeForce 7900 GT/GTO 256MB

I still get up to make a sandwich while I load a new scene ;)

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