The rubber pieces where your thumb sits on the door handle is reccommended to be replaced if you are removing the inner door panel as you can damage the main panel prising it off the alfa intended way. The reccomended method is to drill a hole in the middle of the thumb piece and pull it out using this hole. Its not a big job at all, i did 2 cars (both older non alfas i admit) front and rear doors in an hour and i hadda fabricate a mounting bracket for 2 of the doors as the guy had 13cm speakers to replace the standard 6x9 speaker holes in the front doors.
Ive replaced alot of standard sound systems recently and the factors you need to look at for good fit at are, depth of the replacement speaker and the shape/circumfrence. It wont take alot of searching these forums to find the maximums for these in your 147.
Then the power rating you require is next, whichever headunit you choose find out what all the outputs are for that. If its a 45w x4 which is typical of a standard headunit or cheap replacement then get 50w rms rated speakers, with a good brand these will usually have a max rating of about 150w which is bulletproof. then choose your brand, imo go for sony xplode speakers theyre well priced and there are sizes to fit everything (well that ive come across anyway) for example the 50w rms speaker i fitted the other day were very good quality xplodes and they cost me £30 a pair
Im sure they can be got for around this by yourself if you shop around.
that was the easiest way to upgrade speakers...
if you want more power you will need an amplifier to boost the standard signal, this is a pain in the arse and youll prolly end up with far too much bass if you do it yourself or take it to any local 'professionals' to get it done as it takes quite a bit of know how (and money) to get the sound right, my friend has just had his sound system done properly so that all frequencies are perfectly matched and it cost him £2,000 YES thats £2,000 for a car stereo, waste of money imo i would have a very fine TV and sondsystem in every room of my house for that kinda dough!