Is inner peace after repeatedly performing the الاستخارة prayer and seeing certain dreams considered an indication of good in marrying a particular person?

A vision or a feeling of inner peace can bring comfort in Istikhara, but they are not definitive proofs of what is good. The decisive factor is whether one is guided towards or away from something. If the suitor is a person of religious commitment and good character, repeat the Istikhara. If the marriage takes place, there is good for you in it, by God's will. If it does not, it may be that God has averted harm from you, as Allah says: "And perhaps you dislike a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows while you know not."