The Campden guide is a good recommendation – useful reference source, and as a bonus the sight of it usually makes auditors happy too
This formula from Stumbos’ Thermobacteriology in Food Processing (also cited in the Campden guide) can help you compare equivalent lethality of different time/temperature regimes:
Your Tref is the temperature of your current regime (85°C), T is your new target temperature (65°C), and you’ll need to input a suitable z value for the product(s) in question.
For fruit purees I’d personally also consider spoilage organisms, as these are likely to be the largest starting loading, and whilst not relevant for food safety, for many juice/puree applications they can be the more significant challenge in practice (although this will depend on the fruit as well – pH considerations could be relevant).
Is this first pasteurisation of puree you’re producing, or a final pasteurisation of a product you’re making that uses purees that have already been pasteurised by an initial processor?
The other element to potentially consider, and that won’t really be covered in the normal microbiological comparison of pasteurisation processes, is the implications of a lower temperature on the success or otherwise of denaturing the enzymes that are naturally present in the fruit. At 85°C you’ll have had no problems with this, but at 65°C it could be borderline in some cases. The enzyme composition, and thus the temperatures required to sufficiently denature, will vary between different fruit, so you may need to do a bit of a literature review and/or some trials on this if it’s a relevant consideration for your product range. As with the spoilage organism considerations, it isn’t a food safety concern, but can have an effect on organoleptic shelf life.