i have helped put in many systems so i kinda know what i am talking about
Any speaker is good without an amp...the only problem is you are running off your deck and the most it gives out per channel is 52W (thats the highest i have seen on a deck anyways) I dont think that stocks get up that high... so you compare that to amped speakers where you can get up to 160W per channel...
the only thing you will notice if you do not amp your speakers is really high volumes it will start to cut out...just because it does not have the necessary power to provide to those speakers at high power.
Even component speakers are good without an amp that what i had at first before i bought my 4channel amp. I had cutting out issues like stated so i went to amped speakers...
for you i would recommend going with a component speaker instead of coaxial. It might be a little more expensive but because you are planning to add a sub it will sound better. With coaxials its just all the mid, high, and low notes come from the same speaker. Components only the mid and low come from the speaker and the high from the tweeter included, and the crossover you can set how loud you want the tweeter to be. I recommend these because it creates a "virtual" sound. With coaxials all the sound comes from one area and it is restricted a little bit.
The speakers vary quite a bit. I have friends with Pheonix golds and they like them alot but they might be pricey. I have everything but JL Audio speakers, JL is a top brand and pretty expensive but i think that its only good in its sub and amp world along with their sub brand Total Mobile Audio that is, not in the speaker world.
Kenwood Excelons seem to be good on the market and thats what i have. I have yet to crack them and i am happy with them but i have got the crossovers set perfect.
JBL would be good. you get what you pay for there, and they seem pretty reasonable.
Dont go with any Insignia or Sony or Panasonic stuff. They were ment in the electronics world, and some of the speakers they make are alright but they dont equal up to any mobile audio companies.
I guess all i can say to you is go to an audio store and just take a listen to all the speakers that are there. It all depends on your preference and what sounds good to you.