General Question: How difficult to take a modern commuter/touring bicycle and make it Single Speed?

In general, I am thinking a lot of purchasing a Bianchi SASS for
riding in parks and bike trails in excess of 5 miles for fitness. I
am having second thoughts because I am uncertain whether or not I can
carry stuff on it like water, tire patch kit, pump, food, cell phone,
tools, etc... I'm not talking frivilous stuff here - basically
necessities that any beginners faq would recommend bringing along.

So it got me thinking - how difficult would it be to take a Trek 520
or Breezer or any other touring bike and make it a single speed?
This to me would be the best of both worlds. I have ton a ton of
research but am still a newb. I've looked at custom bicycle solutions
but their seems to be a 3:1 ratio of single speed mountain bikes to
street bikes - and nothing for touring. And that's what I need for
plus a slightly meatier tire.

The commute/touring bicycle would likely be more comfortable on the
long haul.

Any ideas or help would be appreciated!