Downey become Stark. 100% believable.
Spiderman was such a disappointment as a kid... The movie wasn't much better than when he appeared on the kid's show the Electric Company. hahaha
But as a kid I welcomed anything. So to actually see these characters in something that looks real would have blown my mind.
I didn't read Conan comics. I liked the movie and watched it a lot. But we had just gotten cable and the Movie Channel after only having 3 channels for my entire childhood so everything was amazing at that time. And the Movie Channel would play the same movie over and over and sometimes back to back... much to my delight.
I remember my favorite line from that movie... which I haven't seen in 30 years... hahaha
Sage: Conan what is best in life?
Conan: To crush your enemies, see them driven before you and to hear the lamentation of the women.
You mentioned the look of the characters. I think that is what has bothered me most about Tom Holland as Spiderman. He doesn't look like him or sound like I think he should sound. But he has made the character his own and for the most part I like it but definitely not like the comics. The entire cast seems loosely based on the comics too. The names are the same but the characters have changed.