I don't think growing food would be a problem. Yes the sun is further away but it's still bright enough to grow food in a sufficiently modified botanical environment. If you can still harness the power of the sun, you can grow food for free, once the environment has been created. Once the startup water for this system has been installed, it can be recycled indefinitely, for both humans and plants. The beauty of water is that if it's kept inside a sealed environment, it never changes and can be filtered repeatedly. The biggest problems for humans would be physiological (managing illness and ageing) and psychological (living in a container for the duration of life; keeping motivated; living with regret and loss).
This is for course true, but quite temporary. The problem is that all the equipment would eventually break, the habitat will be slowly damaged by the environment, the air will leak and the water will escape slowly from their sealed containers. On top of that, the population will probably grow and you will need to expand your settlement. So, as time passes you would need to fix things, replace things, locate and mine local resources, create local factories, and so on. You would desperately need laboratories to create medicine, because the ones that you brought from Earth will be spent quickly. To make matters worse, the raw materials and their availability on Mars would differ, so you might even need to make an invention or two along the way.
That's hard even on Earth. No country is fully independent, everyone depends on imports of technology, medicine, raw resources or something. Our life would be severely affected if anyone ceased all imports at once, and some are struggling with no limits on them. Mars, which is much more unwelcoming and remote than any point on Earth, would be much much harder.
Bootstrapping an independent civilization that can support itself and build everything for itself is something we can't do down here yet. Building one factory for one small subset of what we need is difficult, starting anew is a whole new challenge that will be enormous even if you built only the most important and used technology from the past that's easier to build there.
On the other hand, Mars will be an amazing learning experience in that regard that might help us learn how to deal with such problems, even down here. So we should definitely do it.