Learn Spanish (Latin America): Me gustaría que compraras una tabla para el surf. - I would like you to buy a board for surfing.

Because the main verb gustaría is in the conditional tense and expresses a wish. In Spanish, when a verb of wishing or influencing is in the past or conditional tense, it triggers the imperfect subjunctive in the following que clause.

Can I use the present subjunctive (compres) instead since I'm talking about a future purchase?

In strictly correct grammar, no. The conditional gustaría requires the imperfect subjunctive (compraras) to follow the sequence of tenses. However, in everyday spoken Latin American Spanish, you might occasionally hear native speakers use the present subjunctive (compres) in this context anyway.

Why do we use para instead of por in para el surf?

We use para to express purpose or intended use. The board is meant specifically for the sport of surfing, making it a destination or purpose.

Wouldn't it be more common to say tabla de surf?

Yes! Tabla de surf is the standard, everyday way to say surfboard. Saying una tabla para el surf is just a slightly more descriptive way to say a board intended for surfing, but they refer to the exact same thing.