The real answer is that you should never try to predict the future behavior of anything. Instead, ask yourself what sorts of risk you are comfortable taking and work back from there.

That being said, the quantitative finance textbook approach would look something like this:

Come up with a model for equity movement - this is typically some sort of brownian motion-based model, with a component for mean (drift) as you mentioned. You will also need assumptions/calculations for other things you mention such as volatility and/or beta (depending on your portfolio).

Simulate a whole bunch of these over your chosen time period. Price your portfolio (simple of you just want one equity) in each simulation. Use your method of choice (Value at Risk, Expected Shortfall, Worst-case scenario) to measure your loss against your risk appetite (first question).

Then, finally, realize that all of these models are technically 'wrong' but still potentially useful for answering interview questions!

The real answer is that you should never try to predict the future behavior of anything. Instead, ask yourself what sorts of risk you are comfortable taking and work back from there.

That being said, the quantitative finance textbook approach would look something like this:

Come up with a model for equity movement - this is typically some sort of brownian motion-based model, with a component for mean (drift) as you mentioned. You will also need assumptions/calculations for other things you mention such as volatility and/or beta (depending on your portfolio).

Simulate a whole bunch of these over your chosen time period. Price your portfolio (simple if you just want one equity) in each simulation. Use your method of choice (Value at Risk, Expected Shortfall, Worst-case scenario) to measure your loss against your risk appetite (first question).

Then, finally, realize that all of these models are technically 'wrong' but still potentially useful for answering interview questions!