
Some breeds are easier than others for first-time owners but that very much depends on the owner, really. Male dobermanns are often a terrible choice but that was my first dog, and he turned out very well - because I was already developing a head for training, I was researching, I had the clicker before I had the puppy, etc. Where it goes wrong is people getting high energy, intelligent dogs (which boxers definitely are on both points) without doing research or being prepared or being patient. It sounds like she is putting the groundwork in which will make a huge difference for her.
What I would definitely recommend is that she finds a good class or private trainer, from the start, to guide her through. In my experience with all sorts of breeds, how the training is done and the life is managed up until pup is about 4 months old makes all the difference. I'm talking general training, teaching them to walk on a lead, manners around people and dogs, teaching an off switch - everything. If I get a call for puppy training over 4 months, almost every single time, problems are setting in (which is why I'm called), bad habits have been learned. If I get a call at the beginning, I am so happy because I know that pup will be learning the good stuff before the bad stuff sets in! Lead walking especially, I see a lot of boxers straining at the end of the lead or on headcollars as they pull hard (and headcollars are not ideal on a brachycephalic dog, so better to train in the first place than have to manage a puller later on).
They do need a lot of input. Very high energy dogs, I find them utterly doolally personally, but in a completely joyful way - really nice dogs. They just love life! As with most energetic dogs though mental stimulation is usually more important. They do need exercise, certainly, but are one of those dogs where someone could easily fall into the trap of giving them more and more to try and tire them out, only to find a fitter and fitter dog that then NEEDS more and more. But work the brain, and they will settle.
Healthwise I'm sure there are people on here more in the know than me but yes, health is a big issue. Heart and cancer I believe are the worst problems but I could be wrong. If I were looking for a pup I would be very careful about looking for breeders who are doing everything they possibly can in terms of health screening, researching pedigrees to select healthy parents.
I've never found them intimidating really but some people probably will do - a lot of dogs can, because they naturally have a forward-heavy stance which can look threatening, and the squishy face and sometimes snuffly, snorty noise due to that facial structure does unnerve some dogs. What I do find is that they are a bit slobbery!