That white block with two wires for the speaker connection is almost certainly just a plug that is part of the factory wiring harness. If you can't get a matching plug for the new speakers, you can just cut the plug off and connect the wires to the new speakers. I'll add that some people have high standards and will never ever cut a factory harness, but I do it when necessary. Make sure you maintain the same polarity, so the wire that now connects to the + terminal stays the same and also wire to the - terminal; the terminal markings are usually on the speaker, but if no + and -, it might be a red dot or other marking almost always on the positive terminal.
I do not like most car sound systems with just front speakers. With fronts and rears, the rears fill the space with more sound which I prefer in a car where imo detailed imaging is not a reality anyway. Plus, you can usually fit larger speakers in back which carry bass better and leave most of the midrange and highs to the fronts.
You can spend hundreds or thousands on the best speakers on the market, but for most people who just want a good sound and not "audiophile" level, shop around on Crutchfield
http://www.crutchfield.com/ to see what pricing and features you find in their speakers, and if you can't get a better local price, their prices are not bad either. You can even put in your car model and year into their car speaker selection page and it'll tell you what will fit exactly into the factory locations ...that's what I did with my old car.