If AENA ends up running Luton, they will manage it in a way to maximise the profit - i.e. maximum fees, minimum expenditure on the airport
That could be said for any company running Luton or any airport for that matter.
Nothing really wrong in my eyes with the Spanish state owning AENA. EDF is owned by the French state but it won't stop them building Britain's new nuclear stations if the price is right.
I would think that the limited company running Luton and not the owners of that limited company will borrow the money on the open market to grow the airport. That money won't be coming from the Spanish government.