An A4 as a first car? This will not end well...
To the OP - the 1.4 Polo is a solid car. Make an appointment at a dealership and take the 1.4 and 1.6 for a proper test drive. If you are happy with the performance of the 1.4 and it's only going to be you in the car, take it (although you do get a lot of extras when you go for a 1.6 Comfortline).
Before I respond, how much are we looking at if a turbocharger breaks?
Phew, ok. Always good to know what we're dealing with based on what people think a replacement turbo costs.

Given all the rather unfounded fear people have of them. Then again if people think turbos cost R30,000 and base everything on stealership pricing, then it's not surprising why there's such fear of them, same goes for 'higher mileage' 'higher end cars'.
A fraction of people know that a turbo costs R5,000 (at least for most VAGs like the one you mentioned, the 1.8/2.0Ts, etc.). Besides the low cost, they are good for at least 150,000 - 200,000km if they aren't looked after too well, and at least 100,000km more if they are looked after (good oil, letting it cool for a minute after pushing it, etc.).
SauRoNZA has already touched on a few aspects. There are so many reasons to get a better, older, higher end car. From a maintenance point of view, one great thing about VAGs is that most are 4-cylinder and therefore cheaper to maintain, and servicing is on par across Polo/Golf/Jetta/A3/A4 etc. R1,500 for a minor service and R3,000 for a major. When I first decided to go higher end, I was surprised to find out that the servicing cost is so similar.
When it comes to the bigger jobs, at around 150,000km one would need to look at a cam replacement (R8,000) and clutch replacement (R10,000, flywheel R4,000). These will last another 150,000km so it really isn't much considering.
There are also decent third party warranties which cover a good amount of work, they go for around R7,000 for one of the better entry packages which will offer a lot of peace of mind. My suggestion of a late gen B7 A4, 8P A3, or Golf/Jetta 5 would leave a lot of money for potential maintenance, warranties, etc.
One of the main benefits you really can't put a price on for a child is the better safety of higher end cars. I would much rather my kid driving around in a solid higher end car than an entry one with fewer safety features.
Having owned two Audis and done a fair bit of maintenance on both, I agree fully agree with SauRoNZA about your assumption. I would never advise anyone to go for a cheaper lower mileage entry car when they can get such better value elsewhere.
The OP should just go and drive cars in a higher segment than the Polo. Chalk and cheese!
One other big reason, a higher mileage second hand car will depreciate much less. There is so much more value and money saving to be had going this route.