I am renovating a farm house built in the late 1800's. I am planning on tearing down the barn from the same era and using some of the wood to finish trim etc. I believe the moisture content is good 7 to 12%. I'm most concerned about bugs. The area is south-western Ontario.
Termites would be the most serious construction risk factor. There are businesses which specialize in the identification and treatment of termite infestation in most areas where termites are common. It would be prudent to consult with such a company if termites are a reasonable concern.
Otherwise, suitability comes down to a question of species and quality of the specific pieces on one side and the applications for which they are intended and the finishes desired on the other.
Working with reclaimed wood presents a few unique challenges and concerns. It sounds like you're going to be using your stock in its raw form so I won't get into milling etc.
The first thing to watch out for is dust. Barn wood is impregnated with all sorts of nasty stuff including molds and animal dander (a carrier of tetanus). Dust masks are an absolute must and take special care to clean out any wounds and splinters you acquire because they will often get infected.
Another issue is imbedded foreign objects. Hitting an old bolt with a skill saw is as dangerous to you as it is to your blade. Pick up a metal detecting wand ($40) and go over your parts before you make any cuts, you'll be amazed at the variety of shrapnel you collect.
Finally, finishing reclaimed wood is its own bag of problems. Most protective finishes don't really like to be applied over top of dirt and contaminates so be sure to start by wire brushing out the worst of the gunk and then apply a good sealer coat before finishing, or better yet use a penetrating oil like Penofin for a more natural look. Note that any finish will change the color of your patina.
As far as insects are concerned, there is a spray that can be applied to the stock that can mitigate or eliminate the threat of transporting them into your house but it's probably best left to a professional to handle it. If you don't see any signs of activity (holes, larvae, etc. ) you might not have anything to worry about.
Put bowls of ammonia in bowls or cans around the wood and then tarp the bowls and the wood so they are sealed up together for five days. When you untarp the bowls of ammonia and wood, all the bugs that might be in the wood are driven out of the wood and they die. I did this with a big ponderosa pine table. I did have a few pine beetles come out, even after that wood had been cut, milled and stored for a couple of years. So that was what I was told to do. Anyway, it works great for killing off boring bugs.
Why tear down the barn? Either way, old sun-bleached barn wood can be impressive without any finish, especially if it's a hardwood like Oak. Powder post beetle holes can also add a distinct character to the wood. If there is active bug activity, they will soon die off. Most wood-eating pests require a source for water and if you dry the wood thoroughly they will either die off or find a more favorable environment.