There's nothing in the scenario described by Xyrik, or the scenario described by Tem42, that says the people involved are doing only "meaningful conscious-expanding profound activities" and neglecting the necessities of life. (Nor, so far as I know, was that the intention of Marx whom you cited as a prominent exponent of similar ideas.)
Trying to do what Xyrik describes might produce a lot of starving-to-death (1) because no one has yet come up with a coordination mechanism better than markets for deciding how much effort to put into what, and (2) because many people will not in fact want to work hard without the prospect of personal gain. But #1 has nothing to do with what you describe here and #2 amounts to considering a different more plausible scenario rather than the (admittedly unlikely) one raised by Xyrik.
There's nothing in the scenario described by Xyrik, or the scenario described by Tem42, that says the people involved are doing only "meaningful conscious-expanding profound activities" and neglecting the necessities of life.
The whole point of the communist paradise is freedom from need. That, as you correctly point out, leads to an incentives problem and a coordination problem. The lack of incentives (which, I think, exists in Xyrik's scenario as the alternatives are... much less palatable) leads to people over-doing pleasant things (meaningf...
How much money would it take to engineer biological immortality for at least half of the world's population, within 20 years, with 99% confidence?