Glock's website says:
Standard Glocks have a 5.5lb trigger pull, while the 34 has a 4.5lb pull. That's just less than a 20% reduction in pull. You will notice it, but it won't be a big deal.
I do believe the rear sight is adjustable, yes.
Sounds like a good deal. I bought my G19 used for about $400, in the same condition (only ~200 rounds fired).