Exegesis:

plēn zēteite tēn basileian autou ‘instead, pursue/seek his kingdom.’ zēteite takes up zēteite (v. 29) and epizētousin (v. 30) without difference in meaning. Hence the clause may be paraphrased as follows, ‘what you should pursue instead (plēn) is his kingdom.’ For basileia cf. on 4.43 and references there. The phrase ‘to pursue/seek the kingdom of God’ means to prepare oneself for the coming of the kingdom of God and try to live according to its requirements.

kai tauta prostethēsetai humin ‘and these things will be given to you in addition.’ tauta, again, refers to tauta panta in v. 30. For prostithēmi cf. on 3.20. Here it has the connotation of giving in addition. The agent of prostethēsetai is God.

Translation:

Instead, or, ‘in place of this,’ ‘as a substitute for this,’ or, ‘do not do this, but.’

Seek his kingdom. If ‘kingdom’ cannot be the direct object of ‘to seek,’ one may say something like, ‘seek the things of (or, that belong to, or, are important in) God’s kingdom, or, where God rules.’

These things shall be yours as well, or, ‘you will be given (or, will receive) those (i.e., the previously mentioned) things as well,’ ‘God will give you those things also.’

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.