Is buying an apartment in installments from an owner, when it becomes clear that it is mortgaged to the bank until the owning company repays the loan, valid or suspicious, knowing that the purchase is for investment and not out of necessity?

What has been mentioned does not entail the prohibition of buying the apartment. As long as you will buy it for a known price and within a known term, there is no sin upon you in buying it. The sin of usury rests upon the owner who contracted and dealt with the bank in that manner.