Prices for dedicated GPS in-car nav system including that of Garmin are down drastically over the last 5 years, and the newer models come with bigger 5"+ screen (vs. our old ones at 4.3") and lifetime "live" traffic & maps updating - and, typically a windshield suction mount, making for easy removal when not in use. Depending on you use the iPad Mini and/or GPS only unit, it could be subject to lots of wear-n-tear and the Garmin is built for moderate amount of abuses including hitting the interior car floor, the mini will likely suffer more.

You can get an good to excellent pre-owned Garmin on eBay for $100 USD or less, depending on features offered, and maps for Japan on a mSD card with lifetime upgrade, for another $30 to $50 USD - that's a lot less than making monthly payments for a Mini. On the other hand, Mini with 4G/LTE data is great as you can use all of its other features when not driving.

For daily commute, I doubt that your spouse need GPS guidance - it's only for driving out of town & over longer distances - my favorite in-car mounting is a CD-slot mounted holder system, good for 5" Garmin, smartphones & even 7.9" Mini or other 7" tablets. It is still a driving distraction but I do use it sometimes, mostly for its voice navigation with turn-by-turn alerts & directions, making the "big" screen somewhat impractical.

Sygic & Co-Pilot are among the alternatives to iOS's Map & navigational featueres, and can be used offline when there's no mobile data coverage. Unless you are devoted to iOS platform, you should look into Android-based Map/Navigational apps on a 7" tablet as they are well developed & incorporate traffic updates in displaying routes & ETA.

It should be relatively easy for any professional car accessory shops to install these GPS unit in the VW & offer a choice in mounting methods. If you do a simple search on Google, you will see examples of VW mounting options & how it was mounted on YouTube.