let y8s walk you through this. cake.
1. price and brand is meaningless.
Something like this: http://www.parts-express.com/pe/show...number=269-184
or whatever size (same brand / product line) is going to be fine. They are BONUS POINTS because they have neodymium magnets and will save you weight!
Unfortunately the $7 awesome miata woofers are sold out.
But the above will do fine for a "decent" install.
head unit is up to you and it largely doesn't matter until you cross the big money threshold.
-------- BIG MONEY THRESHOLD --------
What y8s would do is this:
Find a head unit you like with good preamp outputs
Get a multi channel processor that does crossovers, eq, time alignment, and whatever else.
like this: http://www.alpine-usa.com/product/view/pxe-h660/
it's only 250 msrp and paired with ANY head unit, it'll do miracles. I promise.
Get a multi-channel amp (small class T like the arc audio)
4x75 W RMS minimum. 100 is better. too little power is hard to overcome--especially if you run 8 ohm speakers (not a big deal, they just double your amp power requirements)
get some known awesome speakers that are easy to mount. check Zaph Audio's driver reviews.
* tweeters: vifa ring radiators are great. bonus points if you put them in a small waveguide stuffed with OPEN CELL foam.
Find something with low distortion, a relatively high Qts (.6 to 1.0) and a decent low end response.
You'll do well if you find an XBL based motor system like the old Adire Extremis. There are a few around if you can find them. GR Research makes some. http://gr-research.com/m165xwoofer.aspx
Or something like the Dayton RS series from parts express.
that I've covered before!
rear speakers: you wont need them with the above stuff.