I think everyone has pretty much given you the answer here.
If you are not going to work on the car and learn everything about it, you should NOT get a classic Mustang as your first. Not only because it will be more prone to having issues initially (unless the prior owner took immaculate care of it for 60 years - which is not often the case), but also because finding a mechanic to work on these cars successfully is difficult and expensive. Can you afford to pay $100-150/hr for labor for diagnosing unknown issues from 60 years of random half-assed repairs?
However, if you're going to learn the car and build it yourself, and you are cool with never having any other disposable income or time for a while, then you should absolutely do it! I got my '66 fastback when I was 19 (this was in 2010 so not quite so long ago as some other responses here). Spent a few years building the car into exactly what I wanted, mostly working on the weekends as my car was at my parents' house 2 hours away from school. Some various yada yadas happened but ultimately I put her on the road in her "final" incarnation in 2014, and have since put 75,000 miles on the car, mostly in road trips. I've driven coast-to-coast, all over the US, and even up to Canada. She's incredibly reliable and I have only had to take the tow truck ride of shame twice in those 10 years IIRC - once for a part that wore through after 60 years (acceptable IMO!) and once for a dead coil when I didn't have a spare and I was in the middle of rural New Mexico. But I am the car's only mechanic and know her through and through, so it is easy for me to diagnose and fix problems - I probably do 1-2 upgrades or repairs on the car per year.
So, it can be done, and it should be done, but only if you know you'll put in the work. Those 3 years I spent building my car came at a detriment of my social life and my finances, for sure. If you'd rather spend your free time doing other things, I wouldn't recommend this path for you. Get a fun zippy modern car or something instead.