My advice along the lines of other comments is while figuring out what she likes try to encourage interests in less desirable cars. There are still a lot of deals to be had in the classic car hobby, cars that are still relatively affordable, relatively reliable, relatively easy and cheap to work on, but you need to avoid the cars everyone wants. Depending on the car it can be as simple as looking at a four door instead of a two door, or if she for example shows an interest in something like a Ford Falcon or Corvair or something follow that lead. Just whenever she mentions a car go look up prices, and try to encourage interest in the cars that will be more attainable. Or maybe even get some books or magazines about classic cars and go through them to see which ones stand out as something she would be interested in.

Collectors tend to want the cars that were cool and desirable when they were young, but the great thing about being a younger person in the hobby is that there’s no direct nostalgia for them. Any old car is cool because they never saw any on the road, and they all have charm that modern cars lack. As a millennial that’s why I’m most interested in classic luxury land yachts and station wagons, for decades nobody thought those cars were cool and even nowadays I get people asking why I chose a four door Oldsmobile 98 instead of a sporty car, but it’s because I wasn’t around when those cars were still on the road so to me they’re just cool looking, comfortable, and well equipped cars, I don’t have that memory of the annoying neighbor kid whose dad was a dentist driving one that older people have.

A big consideration that a lot of people interested in getting into the hobby don’t always think about: are you in it for the performance aspect, for the collectible/investment aspect, or just for the vibes? That last one is the sweet spot that will really open up your options. Like a ‘68 Mustang is an unrealistic dream car, especially as a first project. But a ‘68 Mustang coupe, with a six cylinder and an automatic? That’s basically poison to collectors so they’re much more accessible to people who just want a classic Mustang and not necessarily a top of the line performance model. And of course modern cars are so ridiculously overpowered that even a lot of hatchbacks and crossovers will smoke a (stock) classic muscle/pony car in a race, so in the grand scheme of things what’s the difference between a V8 and a straight six? They’ll both feel slow and poor-handling to someone getting their license in the 2020s, recently I discovered that my mother’s Ford Flex has a faster 0-60 time than a Lamborghini Miura.

And unlike pony cars which had exclusive styling muscle cars were just normal midsize cars with a performance package. So as an example if she likes a 442 see what she thinks of a Cutlass, which is the same car with the same styling but without the sporty package or the desirability/price. Make it a four door and the price goes down even more while still being a very cool looking and fun car (and one that people will likely gravitate toward at a car show, because they see restored muscle cars every day but rarely see a normal sedan).

One other thing to consider is safety. No classic cars are really going to be safe, but most accidents are low-speed collisions near home so you can dramatically improve safety in those scenarios with the right car. My personal preference for safety has always been 1969. Shoulder belts were mandated half way through the 1968 model year, and headrests were mandated half way through the 1969 model year (most manufacturers made them standard at the beginning of the model year but I’ve seen some ‘69 Ford cars without headrests). Around those years a bunch of other safety features also became either popularized or mandated, like collapsible steering columns, side impact door beams, disc brakes, radial tires (though that one is obviously super easy to fix if the car didn’t come with them), more standardized exterior lighting, mirrors on both sides, and some other things I’m forgetting. Some brands even offered primitive ABS and traction control systems in the late ‘60s and early ‘70s, though that’s a bit too niche to count on finding. Also in those days bigger cars were always much safer, in a similar way to how pickup trucks today just plow through everything else on the road. A big late ‘60s or early ‘70s car won’t be anywhere near as safe as something from the ‘90s or newer, with airbags and a modern safety cage/crumple zone construction, but it will still be way better than something even from just a few years earlier.