Precision Adjustable Voltage Reference
The 5ppm is only a typical spec, and there is no guaranteed maximum. Also the 35ppm is a typical spec (of total resistance drift). Note that the 3-sigma element resistance drift over lifetime is in the hundreds of ppm so some parts can be pretty bad. You don't see those changes directly in your configuration but they can result in side effects if there are temperature gradients on the chip (from internal or external heat flow) or stresses. 

The tempco of the 'wiper' resistance is surely much, much higher but you say you will buffer that so it may not be an issue. It will also be somewhat sensitive to supply voltage. 

When you get down to the ppm level things like the stress on the die (from board flexing, from soldering, from environmental changes) can affect the value of "semi-precision" resistors like the on-chip parts in question. If it's designed to be a high precision part they can put extra cost in there to compensate (perhaps by using symmetrical resistors that compensate for each other). We don't really know how carefully they've designed that chip. 

I don't see what good a SPICE model would do you for a part like this- you're looking at deviations from the nominal so a calculator and sharp pencil (and a caffeine-sharpened mind) are usually your best tools. 

Bottom line- if you need guaranteed specifications you should specify parts with guaranteed performance. Otherwise you may find yourself trying to make a silk purse of a sow's ear.