If I pick up more chicks from anywhere I may go back to the original breeder of my current birds rather than a new source. They seem to have a level of resistance built up already. I'm hoping that if I follow that route they'd all have the same " chicken germs" and I would lower my risks of bringing home anything new.
At the moment all I have are ideas and notions and I have no real experience yet since I'm still new... So if I'm thinking something goofy I'm totally OK with someone telling me!
It depends on if your main goal is having chickens for you that are healthy in your home environment (yes! good goal) or developing a flock of Marek's resistant birds for others too.
Either goal is commendable. If you're working towards MDV resistance for others, you'd want to eventually add in birds that might have different genes working against MDV or other diseases but by adding them in you'd risk decimating your flock. The result would be fewer stronger birds. And dead birds that didn't make it.
If you work towards strengthening your own flock, you get new birds that have a genetic history of being able to fight off your local diseases. Much less loss, may only work in a limited area. But less burials is a good thing too.
While I can sit here and say-oh do what's best for everyone, I also know I'd get very discouraged and end up doing what's best for my flock. The birds I know.
I'd do the same as you while encouraging those who have different end goals. There is no one right answer.
Now having said that- genetics is only part of it- do all you can to build up good immune systems; good diet, good environment, managed stress*, supplements, and an eye on your flock will help them live to their potential.
* no stress is as bad as too much stress.
