a couple of things to note on the b5 platform... great cars, but the interior leaves some to be desired for in quality or materials... but the big issue(s) for me is the motor... its basically a 2.8l thats been bread for competition(shortened the stroke slightly to achieve 2.7l) and then had two turbos a-fixed to it to force it to make power... well in theory it works fine, but in reality they just took that "girl next door" motor and tuned it into a "super model/premadona/high maintenance bi*ch".... and yes i mean high maintenance it every possible way. i used to work at a smaller independant german car shop/tuner, and we had several of the b5s come in, lovely cars but at about 90-100k the turbos needed to be replaced -that means the whole motor comes out, infact any work on the turbos or back of the motor and the motor is coming out. the o2 sensors when they need changed requires a special tool and some very skinny skilled hands and about 25min of wrenching(per upper sensor) once you understand how your gonna do it and have done it a couple of times. -oh yeah theres 4 o2 sensors too talk about PITA. yeah basically that motor is maintenance heavy, its a race bread motor from the ground up, and although its based off of the 2.8, almost nothing is interchangeable between the two. and parts for the 2.7 are about 2x the cost of the ones for the 2.8. but the silver lining to that is, when they are running and running good they scream and are a hoot to drive, 140 is effortless and feels like about 90 or so, and 90 easily feels like 50.
if you get one and ask me the first mods id be making would be to upgrade the braking system, ie bigger rotors and calipers... not that the car is under-braked in stock form, but its so easy for those cars to start making power and with that power comes speed, that you will soon over drive the stock brakes. and then id go for suspension, leaving power to the end. i have driven several b5s that were pushing over 450hp and 4wheel power slides were easily attainable on surface streets...
i remember one day i was trying to figure out a issue with one "cutting out" some times under hard acceleration, almost like fuel or spark was being cut. anyhow i was going through an itersection making a left hand turn onto a 5 lane road(2 lanes each way, with a center turn lane), and as i was just coming into the intersection(i had the green light to turn) and starting my turn i punched it in second gear at about 20mph... well i used up all 2 lanes and some of the shoulder and didnt get strait again and hooked back up till about 70mph... needless to say i had my adrenalin rush for the day(week). that car was one that had after market turbos, intercoolers, exhaust, stage II chip, and some other stuff... it was a fun car to drive and still would get 20-24mpg on the hwy.
bottom line is they are fun cars but costly cars. i really like the b6 platform other than the motors also need to be pulled at about 90-100k to do the timing chains $3-6k in just parts depending on how much you want to do... a lot of shops charge around $12k for that whole job right there though, hence why you can find b6 S4s all day long for under $10k, but all told id rather have the b6 over the b5, over all the build is a better one and more refined, more power, and way more low end grunt, better sound ect. and its getting harder and harder to find a nice b5 that has not been molested and or abused. and the ones that have not been are owned by purists and they will want a very pretty penny for it. -oh one last thing, stay far far away from the tiptronic in the b5 S4, those 2.7s ate those alive.
fwiw i think your a4 is sexy, and a nice machine, i think if you do "upgrade" to one of those others, you will actually find it a downgrade... you will soon miss the simplicity of what you have now. -lets face it, your a4 is still fun to drive.
