OK man, AF here too, 13 yrs and just shooting straight, so dont get nervous, we are on your side

and certainly not calling you out.
I have just been there done that and most my work on my own toys (after about 10 yrs in our own shop) was done at the Offutt hobby shop as a butter-bar with no money, so trying to be as precise as we can regardless of your budget.
In addition, the way I paid for my 427 powered Mustang was selling FE parts as a new Lt in the 90's, so no kidding, my line isnt BS, I promise
I have also done this exact swap in a Comet at Nellis and I promise, I am giving you good data.
What you did say was the "There is even a chance that it could be a 390. The guy claims it is a 352 because 352 is cast on the block." Thats why I was saying its worth checking, because the casting doesnt mean anything. If I am full of it and you are sure, then so be it, just trying to help.
If you are sure its a 352, then its worth a lot less and harder to sell, and compared to an AFR headed small block, it will be a dog. Nobody wants a 352 unless they need it for a resto
The pile of parts overall
is a decent deal, and the intake alone for an FE is worth about 100-200, no kidding all day long on Ebay. However, even there WHICH Edelbrock intake makes a huge difference, an FE SP2P is worth about 100, a Performer 125, Streetmaster 175-200 depending on condition, F427 probably about the same as the Streetmaster. So specifics matter
The distributor is worth 50-75, so right there it shows its a decent deal, 500 total and you can pull back 200-250 without blinking
The FE heads are probably worth 100-150
but wont sell fast, and a 352 lower end, not worth much, even if fresh, maybe 200 bucks, but you'd sit on it for a while to sell it. Odds are it'll just sit
The C parts and FMX are probably not worth much unless you find a lucky buyer
However, like I said, 2 grand minimum to put it in your car and the T-5 is an issue.
So, would I buy the pile, probably, but
you wont get rich. Would I put it in your car? probably not due to the cost and the fact that a 352 wont make the power I think you are looking for.
Again though, if you plan to build a big FE later, nothign wrong with getting the car setup with a 352 now so later its just an engine swap. All depends what your goals are
If there is any way you can get a crank casting number, we can verify its not a 390.
I know you gave additional data that it has new pistons, etc, which makes sense, but I have seen a ton of rebuilds that are just re-ring and if the guy was wrong, and it was a 390, it would be worth a little (not a lot) more. Maybe not worth more money, but certainly more torque and a better toy to play with in the Fairlane
I guess B/L is, you know you wont lose money at that price, so if you want it, jump on it. BTW brackets, pulleys, etc are worth money, especially if sandblasted and painted, so dont underestimate any of the extra parts on Ebay. Thats exactly how I built a 427 4 speed 70 Mustang on zero budget (first time around)