Just wanted to know what are the problems that the build head can cause. How do I know that it has to be replaced?
In my experience, the build heads don't need replacement unless they've been damaged. Damage to the kinematic coupling, such that it doesn't seat properly on the build arm, would require replacement, as would damage to the printing surface of the build plate. If your build plate has surface irregularities caused by the scraper during the part removal process, you can restore its smooth surface using sandpaper.
If you're using multiple resins, another issue to be aware of is that resin ingress into the build head can cross-contaminate the resin in your tray. Therefore, the best practice is to use a dedicated build head for each resin that you use.
Is there a material that is will not react with these resins that can create a gasket between the aluminum plate and the molded body, thereby preventing resin ingress?
You could unscrew the build plate from the head, apply a silicone sealant bead around the perimeter, and then screw the plate back on. That should create a non-reactive seal.
We'll try to put a quick Instructable together soon demonstrating this process.