How to choose resistance ?

Look up "how to choose resistance on computer. . be specific and you should have answer. Or go to a Physical therapist Department and ask them. they may be able to let you try the different types of resistance. Also depends on the strength you already have....I bought 2 different resistance 3lb and 5 lb bc I knew I would be getting stronger. If you can't handle 3 lb yet. just use special putty, clay. Buy it anywhere specific to working out. No child's stuff.

Can you lock each finger mechanism ?

The fingers do not lock when pressing them down. They go down individually however there is no locking on any point of this device.

which model allows you to move the black grip to the other side?

So sorry I can't help you. I wish I knew.

What resistance would work well for me? I am a young healthy male who exercises daily and wants to improve my hand and finger strength.

This resistance may serve as a beginner for you. I find it gives me the resistance I need without too much problems. As your fingers get stronger then you may want to consider getting a heavier one.