Did someone say my name?!
You can buy any standard headunit and it will fit in the car, however the existing control stalk won't control the headunit and the headunit won't control the display on the dashboard like the standard headunit.
You can get converters for some manufacturers (Sony only I think) which will allow you to use the existing display & stalk with the new headunit.
As for amplified sub, it really depends on what you want. I built my own box and mounted a seperate amplifier in the boot. Or you can buy active subs where the speaker & amplifier are combined into one unit. Maybe worth asking other people in here what they recommend and whree they got it from.
My sub cost about 240 quid overall - £100 for the Kenwood speaker, £100 for the amp and £40 for the wood, materials, wiring etc to get it running.