From the GQ article:

> ... with his former life as a 29-year-old clinical psychologist on the faculty at Harvard named Dr. Richard Alpert. He'd been the pride of an esteemed Jewish family in Boston, with an impressive résumé (gigs at Stanford and Berkeley, a corner office and sterling reputation at Harvard) and all the trappings of success that came with his swinging young professorship, including a blue Mercedes-Benz, a Triumph motorcycle, a sailboat, a single-engine Cessna airplane, a live-in girlfriend in an apartment on one side of town and a live-in boyfriend on the other.

Maybe I'm out-of-touch with Harvard compensation ranges ... but is this practical given solely his salary or would he have need to supplement his income with family wealth or other income sources?

He had family money.

This part is more or less quoted from the talk in the first episodes of his podcast At least of the airplane he then later says ”well the bank owned it”. It was something he bought because Leary insisted they go to Mexico, he was still taking flight lessons at the time.