as far as I know, there is no soundcard, amp, and processor setup that could be done for only 500 euro(but it would be great if i was wrong).
1/8" jacks are fine for a audio setup and will leave more money for a better quality amp and sound card. I recommend using 1/8" stereo to rca jack adapters, so that you can run rca cables back to the amp(s).
the best advantage to using a digital signal(spdif) and processor is that the signal has less chance of picking up noise(toslink cables will never pick up noise). the use of a sound processor is so that it is one less thing the computer will need to do, and should give slightly better sound quality, but might not be very noticable while using the factory wire and speakers.
even if you are still only using the factory speakers, it would be best to run new speaker wire from the new amp to all the speakers.