Learn Turkish: Keşke ekstrem sporlar için teçhizat alsan. - I wish you would buy equipment for extreme sports.

In Turkish, present or future wishes are formed using keşke (I wish / if only) followed by a verb in the conditional mood (-sE / -sA). So alsan functions here as "I wish you would buy" or "if only you bought."

Could I use alsaydın instead of alsan here?

You could, but it changes the timeframe. Keşke ... alsan (conditional) is a wish for the present or future ("I wish you would buy"). If you used Keşke ... alsaydın (conditional + past), it would express a regret about the past ("I wish you had bought").

Why isn't teçhizat pluralized if they are buying gear for multiple sports?

Just like "equipment" or "gear" in English, teçhizat is a collective noun and is treated as uncountable in Turkish. Additionally, because it's acting as a general, indefinite direct object (buying some equipment), it stays in its singular form.

Is it common to use the English loanword ekstrem instead of a native Turkish word?

Yes. While Turkish has words like aşırı or uç for "extreme" in other contexts, the direct loan translation ekstrem sporlar is the standard, everyday term used for "extreme sports."