What speakers to use.

Hi I'm buying a 2000 Nissan Frontier off a family member for a good price in the next couple weeks and in the beginning of planning out a system. The stock stereo is a 6 speaker system 6.5inch in front and back doors as well as 1inch tweeters in the front. I originally thought it was just front speakers so I have had to alter my plans a bit.

I want components up front to replace the stock holes but now I'm confused about what to do in the back. Do I use 6.5's with the tweeters built in? Won't it be hard to get a good match with the fronts or will it not matter? Would it be better to get components for the back as well and find somewhere to mount the tweeters? Before I knew there were back speakers I was just going to put the components up front running off a 2 channel amp with a sub behind the rear seats running off a mono amp. With 2 more 6.5's in the back I can put off getting a sub but I'm not sure what to go with. I'm thinking about going with the Infinity Kappa series but that's not set in stone.

The amp isn't a hard decision for me but choosing the right head unit is difficult with so many features and options. I originally wanted a Nakamichi 400 series head but that was more when my plans were to get a BMW. Now I kinda want MP3 support and with that an easy to navigate display. Thanks for any tips.

My opinion is that you'll want to replace the tweeters up front with those to match whatever midrange you choose.  I've read that component sets are usually carefully matched.  Crossover/ tone/etc.

I'd go the opposite way on the sub/rear decision.  Hold off getting the rear speakers and get the sub right away.  Filling out the frequency range is more important in my opinion than adding 'rear fill' with the back speakers.   You might decide you don't even need rear fill.  I prefer no rear speakers. 

On the head unit, I'm biased, but the Eclipse head units are excellent.  I've also owned clarion and panasonic in the ~$350 range in the past and in my opinion the sound and build quality don't come close.  

In addition to the Kappas I'd demo Diamond, Memphis, MB Quart, and Focal.  I'm sure the rest of the gang here will suggest a few others .. but these are the only I've personally demoed that I'd consider. 

Good point. You will be up front. That back seat is behind you. You wont be able to tell if you have nice components back there as much as the subs. To me it sounds like you are going for SQ and the subs that were mentioned are very good choices. I agree and wont add anymore brands to the mix.