I am looking to replace the factory speakers on my 99.5 B5 A4 w/ Concert HU. I have read and searched the forum without getting the answers I wanted. I have received mixed recommendations from online and local audio shops about replacing the speakers. My current thought is trying BA S45's 4" in the front door and BA S65RC 6 1/2" in the rear at crutchfield's recommendation based on the speakers' low end power range ratings. I am looking to spend only 2-300 dollars. My questions are:

1. Will these speakers sound decent compared to stock? I am not sure of the ohm rating on the stock speakers (3 ohm on front?) and have been told 4 ohm replacements may not have enough volume.
2. should I try BA S55 5 1/4" in the front door instead?
3. If I don't have aftermarket amp, do I need to try to salvage the rear amp?
4. I read the Concert HU has line level outputs. Can you run this to a aftermarket amp and then to the new speakers.

I do not have much car audio knowledge and could really use some help.