She's going to have to be politely reminded that she's not buying new and has a bizarrely strict set of constraints and will likely have to be flexible. The first thing I'd change is the need to look for a specific (non) colour, you want the best example you can find, not the whitest.
In terms of the diesel vs petrol argument. I had my A4 from new in 2013 (so the facelift of what you are looking at, same car, same engines etc) and mine was the 2.0TDIe 136PS model. I did less than 10,000 miles a year (think it averaged to around 8,700) and had no issues with DFP or anything else diesel related. I did however always tend to do a good run once a week which would have been enough to get the DFP up to temperature. Only a handful of times did I park up to hear the fans going nuts because of the increased heat of a regen.