suggestions for an upgrade

im swapping my system into my new car soon and since i went pretty cheap with it(ok dirt cheap) id like to swap some new goodies in. i want this system to be loud but at the same time still be able to adjust my bass enough that i can hear vocals. i have a 1980 benz 280ce, and what i plan on keeping for the new system is my sony head unit and bazooka 3.5" coax's in the dash(i think 50w rms), the hu looks just like a cdx710 but it has the movie type screen. i can never find the model online. i would like some suggestions on what wattage i should go with for the amp. i was thinking of getting a single 10" sub for the trunk since my cabin space is pretty small. i dont need really deep bass but i want it to go loud. anything is louder than the HU amp im used to im guessing. i was suggested to go with any 5.25 comp speakers for the rear deck lid and to power them off the HU. with that i can use my amp for the 3.5"s and sub. are there some brands of comp speakers i should stay away from? im guessing i should go with a 4 channel amp, so would 2 channels into one sub be good, or would two channels into two subs be that much better? i know thats a ton of questions but if anyone has suggestions for the best possible setup for a budget of about 4-500 for the amp, sub, two comps, and everything to wire it, give me some ideas.

thanks for such a quick reply ill look into class d amps, its a ton easier picking from cause there arent too many of them, i want looking at this one http://www.online/CarAudio/ProductDetail.aspx?Productid=18485 is it a bad idea to get an amp thats 4x70w rms at 4ohm when my speakers up front are 50w, would i just have to set a volume limit?

you should be fine. The key is in setting the gain right. Follow the sticky post.   https://www.the12volt.com/installbay/forum_posts.asp~TID~85479~PN~1