It is probably wise to check that they are genuinely not owned which means checking for a microchip. You can buy a microchip scanner on Amazon for about £30 which I think is better than going to a veterinarian to check because it would be difficult if they are genuine feral cats.
However, if they are amenable to be placed in a cat carrier and taken to a vet, it might be useful because when checking for a microchip you can also ask the vet to check their health briefly for any obvious signs of injury or illness.
If they pass that test, once you let them into your utility room and feed them and look after them you will have two more cats for the life of the cats which is a big responsibility but it seems that it is one that you are prepared to take on.
I think you can train them to use the cat flap quite easily by leaving it open and putting food on the other side of it. And then after a while you close the cat flap and they will be motivated to get through it in the usual way.
As they are outdoor cats, I don't think you need to put down a litter tray at the moment. And if when you adopt them, they are indoor/outdoor cats, once again, you, strictly speaking, don't need to have a litter tray for them.
Although, it is said that a litter tray is useful in being able to check cat poop which allows you to diagnose some potential health problems as they develop. But that's a fairly minor advantage to set against the disadvantages of cat litter which are environmental and cost for example. Also, cat litter can be dangerous to cats because of the dust that they kick up and ingest.
Good luck and well done by the way for being so concerned about them. That's very good of you.