Many gold stars for doing lots of homework BEFORE getting a box turtle!
I would say that your assessment that box turtles get sick easily while in captivity is generally correct. I think that this happens for a variety of reasons, the most important being that box turtles have very complex needs that we are all just beginning to understand more fully.
As a result of previous lack of understanding, there are still lots of sources of information out there that are loaded with inaccuracies, and turtle keepers make mistakes in caring for their pets because they've been led to believe that turtles are "Low Maintenance", or "Easy to Care For", etc., when the opposite is true.
Many of the sick turtles you read about have probably been kept indoors under less than ideal conditions: hence the respiratory illnesses due to poor diet and/or lack of adequate humidity, poor bone and shell growth due to inadequate lighting, stresses due to problems maintaining adequate temperature gradients, egg bound females suffering or dying because (in part)of lack of a suitable nesting site, to name a few things.
People that keep their turtles indoors and healthy have, you will find, gone to great lengths and expense to maintain optimal conditions for their turtles, building roomy set-ups, with pricey lighting and heating systems, misting systems, filtration for water, etc., and they tend to monitor their pets regularly, checking for behavioral changes that may indicate a potential problem.
Boxturtles that are kept outside (year round, climate permitting) tend to have fewer health problems because many of their requirements are met by Mother Nature. This is not to say that its less of a responsibility, but rather somewhat less of an investment of time and electricity.
At some stage, pet box turtles will require an outlay of cash: what you need to ask yourself is whether or not you want to spend money on good supplies up front, or vet bills later on.
If you aren't in a position to house your turtle outdoors for at least part of the year, I would suggest that you hold off on acquiring one until you can.
Again, high praise for thinking it through before getting one!
Regards
Stephanie