Actually, it depends on the details.
From the rules : 'If you can honestly say to yourself,
- “Yes, I mainly used Blender to make this.”
- “Yes, nearly all of the foreground models are new.”
- “Yes, most of the models are new.”
then the entry is Pure.’
I think, this definition works fine in most cases.
Regarding textures: Most of the time, using textures is fine. Depending on the project it might be fair to consider extreme cases open entries. However, I still think that usually textures shouldn’t have a large impact on the pure/open categorization. Of course, there might be cases where textures are used in a rather unfair way. E.g. mapping a photo on a plane and calling it a pure entry.
Regarding rendering in Unreal: I think, I’d personally call it open, as this would probably have quite an impact on the final rendering as well as the overall workflow.
As a rule of thumb: If you would feel bad if you won this round and called it pure, then you should probably switch to open. 
Generally: There is nothing wrong with entering open entries. Many great entries have been open in the past. (And many of these have won the challenge.) In the end, it doesn’t really matter all that much.