If you have factory tweeters, than the speaker in the front of your car is not a fullrange...it is a midrange. IT covers all of the middle frequencies, and the tweeter covers all of the high frequencies.
HPF and LPF are filters. HPF filters out all of the sound below the set range (at the amp's specified slope). IF I set my HPF to 80HZ, for example, then all of the sound below that frequency will be faded out to nothing gradually at a rate depending on my amp. Most amps have 12 db slopes....which means that for every frequency interval, the sound drops a 12 db level, until all sound below that point is eventually filtered out.
LPF filters out all of the sound above a set range. You would only use the LPF for subwoofers. HPF is for speakers and tweeters. If you have stock tweeter locations and are going to be upgrading to aftermarket speakers, than you should definitely get a component set. It comes with a tweeter, a midrange, and a crossover. The crossover will do all of the filtering between the midrange and tweeter, but all you have to do is set the HPF at the amp to filter out all low level frequencies (that the sub would need to handle).
I don't know much about the different Alpine amp line-ups, but I do have a 4 channel amp, a component set, and a coaxial set for sale in the classified forum if u were interested.