Does anyone know if it is possible to add an amplifier to my stock '07 MCS system? I am not looking for anything crazy powerful - I just want a little more punch. Later I can replace the speakers or add a sub.

IMHO, for some punch full of clarity and fullness, I would try new front speakers first, and then go from there as needed. Check my sig.

Agree. The MINI stock speakers are terribly cheap and should be the first thing you replace.

Thanks for the advice. So I can begin shopping what size speakers should I buy? What about the OHMs? Does that matter?

Sorry for the simple questions, I know nothing about this.

Start with this thread. Lots of great info especially if you plan to install them yourself. I picked these, check this thread for details of what speakers I got and issues with my install.

Also, shop around at local stereo shops for their recommendations and installation deals especially if you don't want to do it yourself. I just replaced the fronts, and it went reasonably well considering I've never done anything like this before. There was a lot of figuring things out as I went. Could of paid someone $80 or less to do it, but what's the fun in that?

Replacing the rears takes a bit more effort as you well see in the thread I recommended above. Also, it appears not much low freq signal gets sent back there anyway, so unless you start installing new amps and such, forget about it IMHO. If you get to that point, you'll probably be adding a real subwoofer. I just let the stock rears fill the sound stage a bit.

Oh yeah, the standard amp is 4 ohms and most speakers are 4 ohms.

Yes, ohms matter. If you have the standard audio system you want 4 ohm, which is common.

For the door, the large speaker is 150mm and the small is 100mm. However, 6.5" and 4" will fit.

Look for speakers with a high sensitivity rating. Somewhere above 90 db is good. I haven't seen anything rated above 95 db.

It is a lot of work to get at the rear speakers. Try the doors first and see how it goes. Then decide if you want to tackle the rears. They are 6x9"

Read this thread, but note that the speaker sizes in the first posts are wrong.

So what is the BEST recommendation on hooking up an amp for a sub?

I would guess that you need to tap into the speaker lines for the 6" front speakers. Don't use the rears because they don't get much bass.