Agree with what was said above, get her involved in building the car so she knows what's in it and how to at least do basic maintenance and emergency fixes. Then you don't have to be concerned with building the car to never have issues; it's a classic, it's always going to have at least some tiny little thing going wonky unless you do a nut-and-bolt restoration.
On that note I think it's critical to teach HOW to detect problems... like what funny noises coming from under the hood mean or why it's important to keep an eye on the oil pressure and temp gauges. Also doing routine checks on things like tire pressure, brakes, battery etc.
With regards to the engine I'd go with a 318 mainly for the gas mileage. I haven't owned a slant-6 car but from what I've read on here and some FB Mopar groups it seems like a 225 /6 only gets marginally better mileage than a 318 even though it's almost 100 cubes smaller. Also more than a few stories of otherwise healthy slants throwing a rod out the side of the block for no apparent reason... never heard of a 318 doing that. They just slowly wear out to the point where they're burning as much oil as gasoline and have no power, but they still run! LOL
A built 318 with "only" a TRUE 250 HP is quick in a little 3300-lb A-body especially if you have an overdrive trans and decent gears like 3.55s. I think I would do an AX-15 5-speed swap on something like that; they're rated to handle about as much power as a mildly-warmed 318 puts out and the ratios are set up so it works well with taller rear gears like 3.23 or 2.94 (1st-4th are pretty steep, 5th is only .79:1 overdrive). I believe
@75slant6 has an AX-15 in his Duster behind a 5.9 Magnum, they are all over the place and came in a lot of mid-size trucks and SUV's from the 1990s. I have one in the Jeep in my sig btw, almost 250k miles on it and it shifts like new since I've changed the fluid a couple times.
