Will this be okay for my d5100?

[KmH]

I would fork out a little more money for the more expensive one, auto focus and better quality.

I can't do that Will manual focus be okay, is it easy to focus, and easy to do it quickly?

See page 159 of your D5100 users manual - Rangefinder mode. Rangefinder mode is a manual focusing aid built into your D5100.