It is easy to tell DIY fit doors. If you can, get a joiner in to do it, he sould get them all done in a day if everything is straight forward and you already have all the materials on site.
Beware of tracing round the old one because if its been fitted properly the latch edge (and often the hinge edge too if the frame has been clagged up with years of paint) should have been led in and will give a false reading for it's actual width.
I can't believe no one has mentioned marking gauges. I wouldn't entertain chopping hinges without a pair of them, and a saw horse with a notch in the end to hold the door upright for planing.
An electric plane is fine for shooting the edges if you need to take a lot off but I would always finish them with a couple of passes by hand.
Sharp tools are a must. Most brand new chisels won't be sharp, often they haven't even had a secondary bevel put on them, same goes for plane irons.
Oh, and 3mm gaps don't cut it in my book.
Good luck.