OK sorry about the long post but...
RMS is a measure of 'real' power or as we used to say when winding up Dixons sales droids 'kettle boiling power'. If your amp can shove out more than the speakers then you could end up destroying the speakers. Like a good many things power on it's own isn't a good measure. I know of 100w amps running 300w speakers and throwing a cone out. Often a bigger wattage from the amp is suggested than the speakers as the "overdrive rather than underdrive" brigade don't always understand what they are talking about but generally you want the amp better than the speakers. [Those destroyed speakers were Rogers Studio 1's and were driven by a lovely set of £2000 each valve-amped mono-bocks running off huge power supply systems and driven very hard with the 1812 Overture cannons going off...]
The measure of THD (total harmonic distortion) is useful as an amp should be measured across a range and produce a similar amount of power across the entire sound frequency range with little distortion. An example would be a -3db (power range and we can hear a 3dB change) from 20Hz-20kHz at 0.1%THD into 4 Ohms. This would be a good amp.
http://www.carstereo.com/rayfes/ampspec.cfm
For cars 4 Ohms is the expected impedance for both speakers and the amp power. Some talk of bridging amps and 2 ohms etc but I'd suggest starting sensibly (if you want some good reading
http://sound.westhost.com/articles.htm or specifically car audio
http://www.carstereo.com/help/ )
Most external amps will have to be wired in-between the head-unit and the speakers these need speaker level inputs. Some can hook into the head-unit wiring using the ISO connectors to tap the normal head-unit outputs. If you have Harmony, Symphony or most of the single-DIN head-units Rover fitted then this is the way. Some aftermarket single-DIN head-units have line-outs which make things simpler. If you have the High-Line system (screen jobbie) then the amp is separate and can be easily upgraded.
So in direct answer to your questions
the Infinity speakers are nice (I have an earlier set of these lying around that may well go into my 75 once all the other things get done).
An amp should have a good output with low distortion over a wide frequency range (beware +/- 6dB quotes or those that don't bother), not too powerful eg 40w RMS per channel into 4 ohms or 100w to start (Rover Symphony supposedly has 20w but that sounds quite optimistic if you actually want a good quality of sound as well! Didn't know you could get 5 channel amps. I'd probably go for a high quality 4 channel and a separate one for the sub (which can be 300w etc as a sub needs lots of power to move the large cone to give the bass kick).
Keep to 4 ohms for both amp and speakers (the Infinity speakers present 4 ohms to the amp via the cross-over -keeping it simple).
Yes you would need to wire it in but you may be able to get a suitable kit that will allow you to take the speaker-level outputs from your head-unit from the ISO connectors behind the dash and cut in so that the return from the amp goes into the speaker ISO leads in a sort of Y adapter. Can't find one on ebay...
In other words
Currently: Head-unit ->ISO->Speakers
Upgrade: Head-unit ->ISO->AMP->ISO->Speakers
Don't skimp on the wiring for the amp power supply for best results
Discaimer: I was an Electronics Engineer who grew up in and out of Sound Studios with my Father, but car hi-fi is not my area of expertise - Coolcat is