
Hi, good to know you are doing lots of research before leaping in. To start off, pugs are not an easy breed, and to be honest I would probably never had another litter if my first pug litter had been my first time breeding (had been breeding American Cockers for 10 years before started with pugs so had accumlated a fair bit of knowledge and experience.)
Taking one of your points first, and pregnancy and whelping carries risks and the possibility of losing your bitch, it is not common but it can and does happen.
Next, why do you want to breed? It sounds like you love your girl, but from your despcription she is perhaps not an excellent specimen of the breed? If this is the case, and the pug dog is of a similar type, then you are wanting it have a litter without wanting to contribute anything to the breed as a whole. Generally, we try to breed to further the betterment of the breed in some way. I understand that you say she is fit and healthy with good breathing, but the ideal is to have the attributes as well as a quality pug who meets the breed standard. You are at the best unlikely to produce good quality pugs by mating a pair who have common faults, not having seen either dog or bitch and just going by what you write I think you would produce a lightweight, leggy pug, untypical of the breed, although all too often seen in "pet bred" pugs. IF you are determined that your girl has something valuable to add to the breed gene pool then at least you need to find a high quality dog for her who meets the breed standard as well as having all the correct health criteria.
Next, health issues, this is a bit of a hot potato in the breed, there are no KC listed conditions. However, lots of breeders look at issues which they feel concern them and are testing - we think that xrays for Hemivertebrae are the most important. Some breeders are hip testing. It is something to research and decide what is best for you. You also need to know and understand the health issues of the dogs behind her in the pedigree, at least for her parents and grandparents. It is good to know that she has great stamina and breathing, but this SHOULD be standard for all pugs - a pug bred by us is succesfully doing competitive agility, and several others are training for it, you can't do this without good breathing! Just because they have a short nose and may sound a little snuffly, they still need to have the ability to go out and do things.
IF your chosen stud dog is 5 and has never mated a bitch, and both you and his owner are novices, it is highly unlikley you will achieve a mating, in my opinion.
You say you have a whelping box - do you also have the time to bring up this litter? Many pug bitches are not naturally maternal - but even with maternal bitches you need to have the time to be on hand 24 hours per day for the first 3 - 4 weeks.
I am sure other pug people will be along to add lots of other bits I have missed. IF you want more one ot one help or advice please feel free to either PM or email me.