My advice as a comic collector is start with something with less than 50+ years of issues.
That way you can start at the beginning and not miss anything.
My suggeston is read Marvel's
eXiles. The Exiles hop around in and out of dimensions and time periods so it features characters from X-Men, Avengers, Spiderman, Hulk, Fantastic Four... etc. Its a continuous never ending cross-over with a rotating roster. But I suggest this cause you indicated an interest in the classics. The exiles visit points within the Marvel Universe that are considered pinacle to keeping the timestream pure... so classic events like the trial of the Dark Phoenix, the moment the gamma bomb turned Bruce Banner into Hulk, etc. These are all rewatched in the pages of exiles. It'll catch you up on the current on-goings as well as the history of the Marvel universe and it goes into many what if scenereos... like worlds were many of the supervillains plans were succcessful and what could have happened at crucual moments if events went differently.
i.e. everytime a crazy villain tries to do something and is stopped, it creates a dimension where that villain tried that same thing- but was successful.
Exiles explores those possibilities.