Well, from my experience, the links and thumbs on top/mainpage, are naturally the ones getting most visits. So those are the ones you should give priority. You do not need to log in to every sponsor, but with some overall stats tracking, you get the idea. Eventually make something like "hottest" or "top ##", and then sort alphabetically. And new models should definitively be announced on mainpage.
If model A gets 10k hits with a ratio 1:5k, while model Z have a ratio of 1:500 and only 1k hits - then why not place her on top instead? If your objective is "just" indexing, then ok. But for targeted sales, I think my suggestion is better.
Without nofollow, cloaking can be penalized. Besides that, with a dedicated domain, you can centralize the referal codes too for use on other websites
