thats a pretty big amp for such a small car...
many on this forum are very happy using motorcycle amps(ebay is the best source-- usually around $20-50 US, so 10-25 euro)
if sound quality is a more major issue to you, look into class-d amps like the alpine pdx series, or the jl audio hd series.. they are both very small, very powerful amps..
for speakers, i have been most satisfied with the sound of 6.5"-8" speakers. the larger the speaker, the better the bass-- with a 8" coaxial, assuming you can fit them, you might not even need a sub..(personally, in a car like that, i would probably put a pair of 6.5's and a 8" sub, though i am plenty happy sacrificing weight for decent sound quality)
the available speaker locations are really going to be the determining factor-- the amps could fit anywhere there is room. some typical options are under the dash, or in a corner along the side of the seats, or sometimes in between the seats.
i tried searching for them, but couldn't find any pics of the interiors to get a better idea on possible locations..