If you like the 335d, there are a lot of dealers here in CA that use them as loaner cars, since they aren't titled you can still lease them through BMW financial. I'm not positive but they may come with an extended warranty.
If you aren't looking to lease, new car financing still applies to untitled loaners/demos. You just have to comb AutoTrader, usually the fact that they are demos/loaners are listed in the listing.
Since you are probably going to want to drive this second car when with your daughter in the car which means you'll end up driving it too [just for sheer ease of accessability] I would really reccommend getting something you won't dread driving. My Mom had an E65 745 without the sport package that my father hated driving [when we were together as a family, it was obviously bigger than his M5 so he drove the 7] and 6 months after they bought it they traded it in on an E65 750 with the sport package. That's obviously an extreme example, and also an example of a "princess problem" at its finest but we bought both 7's CPO and didn't any money on the trade so we got lucky.
Definitely check out the GTI and check the forums for owner observed highway mileage. Since you mention highway mileage as important I'm guessing this car is going to spend a lot of time on the highway. An economy car is always going to be an economy car, with corners cut, and usually refinement and sound insulation are the easiest things to cut. At least the GTI is an "upscale" version of an economy car. And from one 135i owner to another, a week after I got my 1 I drove a 2011 GTI DSG and was shocked it was a whopping $12k cheaper than my 1, it didn't feel it AT ALL. A new, base GTI should be about $3k more than a Cruze/Focus/Elantra, plus you get 3 years free maintenance.
2016 GLE350/2012.5 991 C2S
2003 E39 M5