PhaedeDI have owned a few of all 3. A b5 s4 is a beast in the snow and a quick fun car to drive on the street. For 16k I would get a 99-01 B5 S4 for like 8 or 9k with 100k+ on it. Make sure its had water pump and timing belt done then save the 6k for miscellaneous stuff you are going to need to fix the car.
The evo is the best choice out of all of them but for 16k you are going to have trouble finding one that isnt beat up or has a ton of miles on it. Plus its going to rape your insurance.
Actually not a bad plan, although be really careful with any S4 especially a b5. Owned my S4 for 2 years and it was a nightmare, however when they run well they are so much fun, and have a ton of potential for mods. Personally, i would rather invest in a Forester XT with an sti turbo and STI front end conversion cause i think they are some of the sickest & most unique cars out there. You don't see many on the road and its something you can truly make your own.