In a 100% socialist structure, the owners would be faced with what challenges?

Key features of SOCIALISM

Public ownership
Lack of Price mechanism
Non-existence of competition
Planning
Work according to capacity, eat according to necessity
Non-existence of Markets
In a pure socialist economy, there are no private goods. No one pays for anything, but everyone uses them. (This is an impossible situation in reality because; some goods once consumed cannot be available to others).

The ownership of all goods is with the state (common ownership).

The dilemma of the owner will be then:

What to produce,
How much to produce
To whom to distribute the goods and how much
Who will be employed to produce them ( how many to be employed)
What resources to be used for how long and how much
In fact all the decisions that a market based economy automatically decides through the mechanism of supply and demand, will be missing. The all knowing planner will have to do all the work of all markets, and work-out the interrelationships between all these markets and will have to know the past and future, sustainability of such an economy. By no means a simple task!