What diseases should I worry about in my small flock?

I think they most important factors for health and feed and housing. Feeding a more expensive, higher protein feed is worth it. The housing should be disinfected yearly (usually in the spring), and ample room should be given to the birds. Keep your coop well ventilated in the winter, even if it means the coop goes below freezing.

Those might not sound like the typical "bio security," but just having solid, healthy birds is the key to preventing disease. Once they get something, it is nearly impossible to recover.

I had a nasty disease that killed about half of my 36 bird flock, and we had no idea what it was. Although the worst of it was about a year ago, I sill occasionally lose birds to similar symptoms. We did a necropsy (a year ago during the worst effects of the disease), two birds were infected with overpopulations of commonly existing bacteria that don't do damage unless they overpopulate. The third had a relatively unknown disease that was similar to Marek's. My point is that something got them sick, weakened their immunity, and from their they got sick for various diseases that never truly went away. If they had just been in good health to combat that starting sickness, none of the snowballing would have happened. 

As important as it is to disinfect boots when returning from other chicken farms & not integrate new birds to an existing flock (this could introduce new disease - even if the new birds are healthy, they could have on them a bacteria that they grew immune to, but your existing birds don't have immunity to it. Plus the existing birds will be under stress and be more susceptible), nothing will prevent disease like having sound, strong birds to start with. 

Keeping a closed flock is your best bet. I've read horror stories in the emergency section about people taking their chickens to bird shows...then a few days later their birds are wheezing, sneezing , bubbly eyes etc...

Some diseases go airborne, some are carried on clothing and shoes. Petting someone else's bird can bring dire consequences when you handle your birds afterwards. Keeping everything dry is key for good flock management as well. Dont introduce new birds to your existing flock, quarantine them away from your flock at a good distance for at least 30 days. Most diseases will show themselves during that timeframe. This will also give you time to visually inspect birds head to toe. Consider dusting and worming as well.