The word wherefore is an antiquated way of asking "why?" This can be either לָ֫מָּה or מָדוּעַ. The former is far more common, so it's more likely that you would use it.
Weingreen tells us that speakest in this context should be in the imperfect. You might think of it as "would you speak" in modern English. "Why would you say such a thing?" This gives it the modal / irreal sense. Thus is also provided to us by Weingreen in his notes. The phrase should read לָ֫מָּה תְּדַבֵּר כַּזֹּאת.
Weingreen tells that thy wife, Sarah should be rearranged to say "Sarah, your wife," which is שָׂרָה אִשְׁתְּךָ, since this is the more common order in Hebrew.
Will bear a son is simply the imperfect of the verb יָלַד. As a first-yod, it loses the yod in the imperfect. In this case, the 3fs is תֵּלֵד, since it takes Sarah as its subject, and the direct object is indefinite: בֵּן. Remember that the construct with makaf becomes בֶּן־ (with segol), but without makaf, it is בֵּן (with tsere). There's no reason for it to be in construct here. However, the possessive / indirect object is expressed with לְךָ.
Putting together all we have so far:
And Yahweh said to him: "Why would you speak like this? Sarah, your wife, will bear a son to you."
Bless is the piel stem of the root בר״ך (bet-resh-kaf). Think of it in terms of "knees" (בִּרְכַּ֫יִם; sg. בֶּ֫רֶךְ) and "blessing" (בְּרָכָה). Like שִׁלַּם "he paid," it tends to take its 3ms qatal with patach (בֵּרַךְ), and notice that the tsere under the first radical is due to compensatory lengthening by the rejection of dagesh in the resh. The 1cs form is בֵּרַ֫כְתִּי "I blessed," as should be expected. To make it future (predictive), add the vav: וּבֵרַכְתִּי "and I will bless" (it is וּ because of the BuMP-Sheva rule). To add the direct object "him," you can either use the explicit אֹתוֹ (the DDOM with ו for 3ms) or just add the ו to the verb and get an attached suffix: וּבֵרַכְתִּיו and I will bless him.
Altogether, we have:
And Yahweh said to him: "Why would you speak like this? Sarah, your wife, will bear a son to you. He will walk in your ways, and I will bless him."