I think the original poster specified a span of 96-99. But big improvements were made in 00 and 01. The A4 got a facelift in 2000, and the 1.8T got more hp in 2001.
In addition, a '99 would be off warranty, while the '01 would not (maybe.)
IMO, the extra spent for the '01 would be worth it, from a standpoint of 30 extra HP, fixes to the front susp. (IIRC, again) and that the dreaded timing belt change is not quite due yet, while a '99 might be overdue, to the tune of about $1000 (or, if you wait long enough, $5000 or so.)
But if there's only $13k to spend, don't spend it all on the car purchase. You're going to need to do the t-belt (probably) and brakes (probably.) If you have that done at a stealership, I'd guess (conservatively - you could most likely get these things cheaper) that you'd be down $2000. At an independent shop, you would most likely get a much better deal.
That would be part of my calculus, which might put the '01 in reach. Something to think about...
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