simplest solution for a mini is:
4 channel amp (say 4 * 100W)
2 channels for the front speakers
bridge the remaining 2 channels for the sub
A small sub (8 - 10") in a sealed box in the boot
split/component speakers up the front
the tweeters - I've had best success in compact installs aiming the tweeter at the opposite side passenger, so drivers side tweeter point towards passenger, and vice versa. High frequencies are very directional so this helps with staging (centralising the audio).
You don't really need to worry about impedence. All car speakers are 4 ohm. If you get a sub which has two voice coils, they will hopefully be 4 ohm each. then you can run each voice coil in parallel which gives a 2 ohm load. if you do this make sure your amp specs state it is stable at 2 ohms.
RMS - think of this (but don't) as the 'average' power. Cheap equipment usually states max power, because bigger numbers = better sales. Ignore these, RMS is all you need to know.
Your front components will want to be around 70WRMS.
Edited by sixtyeight, 31 October 2014 - 12:58 AM.