I hope Su.Ki. will respond to your question, however I have a simple explanation why "had" does not work in your sentence:
"If I were rich, I would buy a house by the seaside." CORRECT [would buy shows intention to do something]
"If I were rich, I had buy a house by the seaside." INCORRECT [using had can only be done if the verb tense is the past perfect tense, in which case the proper past participle of the verb buy is "bought". However, the past perfect tense is not used with the conditional form "If I were...".]
No - not always. The contraction 'd can represent either 'had' or 'would'.