Get some thin front component speakers, i recommend the focal range, they have several different models to suit budget ranging for £150-400. Forget the rear speaker as you dont really need them, a car isnt a best sound stage but and you should only have mid and bass coming from behind. This will also free some budget up. You can get some fairly nice decent quality amps that can fit in the glove box behind the passenger seat, sky is the limit for cost but a four channel will probably set you back in the region of £200-500. For the sub you can get a free air type that can replace the bose one behind the drivers seat or just get a box that fits between the boots under the rear beam. These boxes are often on ebay from the states around £150 and can take either 1 x 10", 2 x 10" or 1 x 12". Only downside is the box capacity is fairly small so you would need to shop around for a sub/s that would be suitable. I ran this setup with 2 x 10" JL subs which were ordered in from the states by my local audio place and they were about £170 each. Also add the costs of a amp wiring kit to the list about £30 and to do it properly replace the speaker wire to the front speakers with something decent probably around £15.