ἃ καὶ πάντων, κ.τ.λ. ‘And, just for that reason, nothing is more surprising than that you elect’, etc. ἅ, acc. referring to the whole preceding statement, lit. ‘and as to these things’ (like quod before si and nisi): cp. Thuc. II. 40 § 3, διαφερόντως γὰρ δὴ καὶ τόδε ἔχομεν ὥστε τολμᾶν τε οἱ αὐτοὶ μάλιστα, καὶ περὶ ὧν ἐπιχειρήσομεν, ἐκλογίζεσθαι: ὃ [acc., as to which thing, ‘whereas’] τοῖς ἄλλοις ἀμαθία μὲν θράσος λογισμὸς δὲ ὄκνον φέρει.
προχειρίζεσθε ‘elect’, lit. ‘make ready for yourselves’: cp. [Dem.] In Aristog. I. § 13, ὁρῶν ὑμᾶς κατατάττοντάς με (designating) καὶ προχειριζομένους ἐπὶ τὴν τούτου κατηγορίαν.