It might be cheaper if you purchased a single month subscription to MAX and or Maya, studied a little on how to use the Arnold shader to make the needed materials, and then did the work yourself. Plus you would have personal control over the materials and renders, of course. You could expect to spend half of or a whole day learning the Arnold renderer. Professional-grade knowlege would take longer than that, of course, but your objective simply is to offer your models in MAX and Maya file formats. People purchasing your model in those formats are likely to be tweaking the materials, anyhow, and probably using another rendering engine such as V-Ray, ... so you wouldn't really need to become expert in Arnold. You might not be aware that you can "rent" Max and Maya for just a single month, but it is easy to do so, and relatively inexpensive. A month is more than long enough to learn to use these softwares at the minimal level you need, and to convert all your models.
