If you are getting a filly, you will not need the 6' high fences. I believe for horses 2 years and under you only need 5' fences, which is pretty standard.
Regulations are one thing. Being sensible & safety conscious is another. Just because a horse is under 2yo doesn't mean it can't/won't try to jump a fence to escape. If the horse(whether 'mustang' or just 'unmanaged'(there are a lot of 'station bred' horses here who would not see fences...) is not accustomed to fences, any wire fence - as opposed to big, solid... - can be dangerous, as the horse may not have the experience to know they can't try to go through/over them. And it also depends on the way you handle them - if you're going to terrify a horse into trying to escape, like I've seen CA do to newly caught brumbies for eg(not that I advise this, in a pink fit), then you will need 6' solid fencing.
From what I've seen and experienced it's a lot of work having a mustang. I'm not just talking about the training. They can also have health issues, ... I got it to clear up finally, but it took me several months to figure out what he needed. He also has issues with his hooves. He has high low syndrome, which makes him difficult to ride at any higher level ... Due to how long my horse has had this unmanaged I'm not sure if I will ever be able to resolve it completely. ... I don't think I have seen a mustang who didn't have some high low/club/turned in or out feet.
Apart from the initial setup & getting them used to fences if they're not, I don't see any diff with brumbies(granted, maybe some lines of mustang are different, I don't know, but I doubt it) as with any other untouched/ungentled horse, in training or in health/hoof dept. Many, many domestic horses have diarrhoea & other probs, due to feed & management probs, that can indeed take a long time to get to the bottom of. Many, many, many domestic horses have 'high-low syndrome', or otherwise imbalance etc, due to body issues or incorrect/insufficient hoof care & management. Due to the chronic-ness of issues, how old the horse is etc, it may also be just as unresolvable in domestics.
In essence, a horse is a horse, whether he was born wild/feral or domestic, and generally speaking(aside from odd genetic defects), a 'good' horse(behaviour & health & soundness) is MADE, not bred.
On that note OP, people often go on about the wonderous hooves of mustangs compared to domestics. This *may* be so, depending on the environment & lifestyle, and age of the horse. As horses who do many miles a day over dry, rough footing tend to have well 'maintained' and strong hooves. BUT even in ideal situations, horses don't *begin* to develop good hoof strength until they're around 4yo, and horses that have grown up in 'cushy' environments, necessitating little exercise etc, will not grow good feet. And even horses who have grown great feet, having grown up 'on the range' are at risk of becoming 'poor footed' if they are managed badly, in a bad environment. \
Case in point is one of mine. He was a late-caught brumby(think he was around 6yo when caught) who had AWESOME feets when I met him. True 'rock crunchers'. Big, robust digital cushions. But when I met him he was living in a rich, damp, soft, ryegrass & clover paddock AND was fed twice a day & was fat as mud. I warned the owner of the risks of how she was keeping him. I trimmed for her for a while but then she sent the horse away for training & I moved.
It was a fair few years, before she rang me last year to say she needed to rehome her horses & she'd love me to have one. He'd been moved finally to a better environment, because his mate had finally foundered badly - I was told they had both lost heaps of weight since, but they were both HUGE! I was shocked to see the state of his feet - not that I expected them to be great, I'd warned her years ago & it was only 6 months ago she'd moved them, but he was supposed to be getting farriery from a rehab specialist - among other things, his heels were 2" high!! So... I've had him for a year now, and he needed boots for the first 6 or so months when ridden on rough ground. He has finally got back to a state pretty close to when I first met him(except for his obesity back then). We just got back from a 12km ride on sharp gravel('blue metal'), where they did a fair bit of trotting, some cantering, and he never put a foot wrong, his feet hardly showed any wear by the end even.
I've seen people with no horse knowledge train both mustangs, and baby horses. Not sure it was the best for either party, but it can be done.
Just because it *can* be done, doesn't mean it's a good move. Way too much risk to the horse IMO, not to mention potentially to people involved too, if the horse is inadvertently made dangerous - too easily done. Not that I'm assuming OP has no knowledge, but IF they don't, I would advise against getting a young horse, regardless whether feral or domestic, find a well trained one, AND a good trainer to help teach them.