I didn't see your post about the youtube you watched when I posted my thoughts on undercoating the Prime above. Have you gotten a quote from zeibart for the same service if you bring the Prius to them after you buy it? My take is that there is not much difference in the effectiveness of undercoat if put on a new car or a 2 or 3 or 10 year old car. There are some advantages either way, having it done on the new car or waiting to have it done, it just depends on you personal preference.
Ziebart quoted $699 for undercoating and rustproofing. Ouch. Toyota dealer quoted $599 for the same Ziebart service. My budget is kinda tight so i'm leaning towards no rustingproofing/undercoating for now. Nextsummer i might have it done or i might do it myself. I see the point of not having it done at all but i plan on keeping this car for 10 years, maybe more...and they really salt the roads here in western NY.
From my experience on owning cars in New England, even untreated, most modern cars will be fine for the first 10 years. Surface rust will develop as soon as virgin steel sees moisture and salt, but rust has never caused any mechanical or structural problems on my cars less than 10 years old. The rust becomes problematic after 10 years, and so far I have never had a car lasted longer than 15 years on NE roads. But that's only two cars (not Prius) lost to rust caused problem out of 13 cars I have owned in the past. I did not keep other cars that long.
Piece O Mind = Drive around to different Independent car repair shops and offer ~$20 to ~$40 dollars for an hour on one of their unused lifts. Then spray your brand new car Yourself Before It Sees Any Salt. www.amazon.com/CRC-06026-Heavy-Corrosion-Inhibitor/dp/B07N1VXTVW/ref=sr_1_6?crid=2SDYTAKB0A9T3&keywords=crc+rust+inhibitor&qid=1576684066&s=automotive&sprefix=crc+rus%2Cautomotive%2C387&sr=1-6 Rob43
In my purchase situation, the price difference was $2000 between the 2 trims. For me it was no brainer to pick the XLE.