Information on visiting Iceland - tourism [closed]

I am planning to travel to South Iceland for 3 days and 4 nights in February or March 2017. I want to do the following:

1. Jökulsárlón Lagoon Tour i.e. just see the glaciers floating on water from a distance (I am not planning to do any glacier walk/ excursions or things like that)

2. See the northern lights

3. Bathe in the Blue lagoon

4. Golden circle tour

Among the above four, the first two are my greatest priority. I would like to know about the possible budget that will be required for each of the above four items. Also how should I book the trips? Is it possible to book the trips upon arrival on any hotel in Reykjavik? Or is it compulsory to book them online prior to arrival in Iceland? Also please shed some light on your personal experience if you have already visited South Iceland. Other Comments or advice are also welcome.

EDIT for the last paragraph

Among the above four, the first two are my greatest priority for which I would like to know the best time to travel to Iceland. Also I would like to know about the possible budget that will be required for each of the above four items.

Thanks for answering.