Commercial / Legal use of emulators

Hello,

I am thinking about configuring a few emualtors (MAME, virtuanes, zsnes, fusion, psx, VisualBoyAdvance, Project64) to the same controls and through a front end and / selling the PCs commercially.
I know that ROMs can't be given out but am wondering about the use of emulators.
I've tried to find a couple of the official sites to inquire / read up on but there just seems to be download links everywhere..
All the forums I could find were pretty old threads and in the gray area..
Are there any fees / donations / permissions that I'd have to ask to use the emulators?
Or if they're open source it's okay?
I read somewhere that MAME cannot be used in a commercial product. Several front ends I have seen provide direct / automatic downloads for MAME and other emulators. Is this their way around it or are they legally allowed to be provided?
If emulators cannot be provided, can configuration files?

Thanks.