it really depends on what you want to do.
I've swapped out the stock 16" for 18"'s too. Think i may have been the one who told pezza too. But BMW wheels are the same PCD or stud pattern so these bolt straight on. You'll need so hub centric rings too, think mine were 70.3 to 72.6 mm ones to centralize the bmw wheels on the camaro hubs.
The only tires I ever got on my 16's were the cooper zeon zxs at the 245/50r16 size and they seemed pretty good. But for 100 a tire I prefered to get sencond hand alloys. Currently run falken fk452's at 105 a tire on the 18" in a 255/35r18 size and really happy. You'll have to weigh up the cost options really. You can pick up a decent set of alloys with parts worns for less than a new set of boots for the 16's
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'99 Z28 A4, 253 LS2 heads, 228R TSP cam, Patriol Dual Springs, Double roller timing chain, LS6 ported oil pump, OBX LTs, 3" True Duals, LS6 intake, 42lb/hr injectors, SLP lid + bellow, EFIlive COS 3 Speed Denisty Tune (self tuned), 3200 Circle D stall, CAI mod, KYB AGXs, Eibach Pro Front Springs, UMI panhard, LCA + Brackets, Strut Brace, Custom Subframe connectors, WS6 axle with 3.42s, 18" American Racing Rebels, US mirrors, 93 tail lights, 3.5 inch custom cowl hood. Mickey T's Street DRs on Vette-ZR1 wheels for the strip !
Best ET: 12.0 @ 116 mph (on a 0-60ft of 1.8 s)