Its all down to cost. 15 track tyres are a lot cheaper and more available than 16s.
Almost any OE renault 15" wheel will suit track purposes. But most go for aftermarket as you can get lighter options, which is better for chasing 10/10ths, but for general trackdays wouldnt even be noticable.
4x100 PCD wheel is what you need, but just as important on non renault wheels is centrebore, this is 60.1mm on renaults and larger than most other manufacturers (vauxhall, vw etc which share the 4x100pcd) Most aftermarket wheels will have a larger centrebore (circa 70mm) which just need cheap plastic spigots to space to 60.1mm.
Generally 15x7 with an ET between 30mm and 50mm are good, the lower the ET (offset) the further out they sit. So going for an ET around 35mm means you shouldn't need spacers to fill the arches and get widest track.