I can't answer all your questions, but I've been on azathioprine for a year and the only real side effect I've experienced is hair loss (not loads, but I notice more coming out when I brush and wash my hair).
You will be more susceptible to colds etc because azathioprine is an immunosuppressant, but everyone reacts in different ways. I haven't had a problem with this even though I travel by tube and work in an office - only had one or two colds in the last year.
Having said this, azathioprine alone hasn't really worked for me (while on it I had a huge flare and ended up on prednisolone short term and now infliximab). It's a big decision to switch medications so make sure you talk through any concerns with your consultant.
Diagnosed with Crohn's disease in Sept 05 aged 24
Resection in Nov 06
Currently taking Remicade, 150mg azathioprine, B12 injections, iron, multivitamin, probiotics and omega 3
Previously taken Prednisolone, Entocort, Flagyl, Cipro, Pentasa