The diagnostic port should be under the knee bolster on the driver side (atleast on my '97). You do not need to remove any trim panels to access it. If it is in the ash tray area there may be a small cover over it.
As far as the oil light, the oil pressure sending units are known to go bad. Its located on the passenger side of the engine on top of the oil filter housing. here is a link to a replacement sending unit from autozone: Oil Pressure switch
Also try changing the oil and filter (I use the Mann filter). For some reason on my '97 when it gets close to oil change time the oil light will come on at low rpms/cruising. Change the filter and oil and all is fine until the next oil change.
Can't help you with the rev counter. Hopefully someone else can help you out with that.
There's some good diagnostic software specifically for VW/Audis at www.ross-tech.com
They have a free version VSDS lite but you will have to get a cable from the ebay or the like. They also have licensed versions that do much more.
Also check out Audi Pages
for lots of technical articles and maintenance how to's. Under the oil cooler replacement tab there is a picture showing where that oil pressure switch is.