This is actually a great service in Montenegro. Then again, depending on the country you are in, if you try to pull out more than about 500-1000 dollars/euros per day from ATMs you tend to hit roadblocks that are put in place by the local ATM networks, so it doesn't matter what your theoretical limit is, you get blocked.
As for buying the Porsche, yes it could work, but it is not that simple. Every bank has anti-fraud software that detects unusual movements, and so do the networks (Visa, Mastercard etc). So if you regularly make large POS transactions that will be fine, but if you take a completely new
debit card, load it up and try to buy a Porsche without informing your bank in advance, it is highly unlikely to work.
In my experience most people who want no-limit debit cards want them mainly for cash withdrawals, which also raise extra red flags.
Usually credit cards or charge cards or corporate cards like Amex, Visa Corporate etc have a much higher spending tolerance than debit cards. So if you want to make large POS purchases, an Amex card will suit you much better than a debit card. Supposedly, someone once bought a private jet and paid with an Amex card. But then again you cannot draw cash with Amex.
So it all really depends what you are trying to achieve.
