If you already have a mountain bike I would say use it for now.

Use some of that money to get it checked out by a bike shop, cleaned, lubed the whole deal, and buy some slicks(tires with no tread).

Helmet, anything that is approved will work(forget the certifying agency).

Wetsuit, any wetsuit will work, thinner is better, like a 2 or a 2.5mm. Swimming wetsuits are upwards of a few hundred bucks and you would be better to wait to see if you really want to get into the sport first.