In the sentence 'Once we rent the apartment then we can start considering a house and deciding where we'd like to locate' should there be a comma after the first clause?

Yes, you need a comma, but you don't need then. And locate (relocate?) is not a good verb.

The sentence could be punctuated: "Once we rent the apartment, we can start considering a house and decide where we'd like to live."

You could also rewrite the sentence with the phrases at the end: "We can start considering a house and its location once we rent the apartment."

(after we rent is also better in this form)