I assume you already know the particle ば for condition ("if", "when").
You don't have to think of it as a special construction. To understand the difference, just imagine what a typical answer for each question would be. It should be a simple grammatical operation; just replace どんな with something, and drop か:
→ 優しい人がいれば、結婚します。 If there is a kind person, I will marry him/her.
→ 優しい人と結婚します。 I will marry a (certain) kind person.
So in the first question, the questioner doesn't have an assumption whether the other person will ever marry someone. Or the questioner may be thinking he is reluctant to marry someone (i.e., "I know you don't to want to marry anyone, but if you would marry someone, what kind of person she would be?").
On the other hand, in the second question, the questioner already knows the other person will marry someone in the near future, and is just asking what kind of person he marries.
Unsurprisingly, regarding どこへいけば, it is used when you expect an answer like 秋葉原へ行けばパソコンが買えます, 京都へ行けばたくさんのお寺が見えます, etc.