What is the ruling on thinking and worrying about an unseen second wife, and whether she will be like the first, after seeking Allah's guidance (istikharah) and proceeding with the marriage?

It is recommended to look at the prospective fiancée before proposing to her. This is considered a recommended act by the Hanafis, Malikis, Shafi'is, and some Hanbalis, based on the Prophet's (peace be upon him) saying: "Look at her, for it is more likely to engender affection between you." So, if you intend to marry a woman, look at her and inquire about her religiosity, morals, and character from trustworthy individuals. If you are pleased with her, then approach her guardian; otherwise, leave her before the marriage contract.