Passats have a very well known problem with blocked plenum chambers leading to leaks to the passenger footwelll where under the carpet and prone to soaking there is an expensive bit of electronics controlling the windows, door locks, alarms and some other functions. Not sure Audis have the same issue.

water ingress was a problem with both A4 and Passat of that era. Pollen filter housing seal and blocked drainage holes under battery. Leads to lots of water under the footwells in chassis recesses, controller for various consumer electrics like alarm, windows and so on get immersed and then starts playing up. I had the issue, alarm going off at randomn, windows opening on thier own! VW no help as out of warranty, took two days to stip the interior and dry it out and what seemed like a coupe of gallons of water drained out of chassis when I punched a hole in in a plug under there. Fixes are DIY and will be fine if done right, but I would walk away from anyhting damp as I would imagine electrics wont stand for long term submersion. Check all footwell (front/rear) carpets for damp, press down hard and feel for damp, feel for areas that indicate a recess, press those particularly. There are pages and pages on this topipc online, how VW ever got away with it is beyond me, my local delear said they'd never heard of the problem!

My experience is that an A4 is set up for a noticeably more sportier drive than a Passat. I've driven an A4 for a month long rent and owned 2 Passats and would choose the Passat for comfort as most of my driving over the last 15 years has been motorways and some town and the Passat on 15" wheels was brilliant over the lumps and bumps and good on the motorways. The A4 on bigger wheels and sports suspension (most are) will be a more engaging drive (to be honest anything will be over a Passat) but you'll feel every bump.