Good luck. You'll need it. Guaranteed headache.

Will you design the amp from scratch or use different DIY stuff from eBay?

I know quite a bit about power electronics, and generating a higher voltage from 12v with substantial wattage, is not easy! Especially because you would need more than one voltage for your control.

If I were you, I would buy a 12V boost module from eBay. Go with something physically big, they overrated power a lot, and neglect sufficient cooling..
Then find a class d amplifier module, and combine in a box. Add additional decoupling after the boost supply.

>>1001913
Probably going to get kits from eBay as needed if they end up being cheaper.
What do I do about going from 12VDC to a bipolar [some voltage]DC as required for the amplifier?

>>1001920
It sounds like you know little to nothing about electronics.

Search eBay for amplifier modules, they come assembled, and handles voltage conversation part for you. You only need to supply a single voltage of 24 to 60V. If it has a lower input voltage, it is most likely not high power enough..

To increase the voltage from 12v you would need a boost voltage regulator or step up supply. Same thing, two words..
This needs to have a wattage rating high enough to supply your amplifier. I guess 100W minimum, as they tend to be rated higher than they actually are..

Search eBay for class d amplifier module, or class d amplifier board. Find a mono or bridged board of 100watt or more. Then find a boost converter capable of supplying power and voltage for the amplifier.

>>1001907
save your self some time , and it might even be cheaper, get one of these http://store3.sure-electronics.com/audio/audio-amplifier-board/medium-power