How to create content AND register a user at the same time?

Hello,

I'm using Drupal to create a fitness directory containing a list of fitness centers, personal trainers, fitness equipment... I have created a custom node type for each type of entry ("fitnesscenter", "personaltrainer", "fitnessequipment"...). Anyone can submit an entry in the directory, but they need to have a Drupal user account to do so.

Question : How do you replace the 2-step process of (1) registering a new user account into Drupal, and then (2) creating a new entry in the directory, by a single form where the new users can both register AND submit their entry at the same time?

Here's what the forms would look like:

FITNESS CENTER:
- username
- password
- name of fitness center
- address of fitness center
...

PERSONAL TRAINER:
- username
- password
- name of personal trainer
- rates of personal trainer
...

When the form is submitted and validated (username should not be already used, etc.), the system should 1) create a new user account, 2) create a new node with the entry details, 3) attach the new node to the new user account.

But I'm at a loss how to proceed here. At first, I thought I'd customise the user registration form, but it's impossible because 1) I need different customisations (one for fitness centers, one for personal trainers...); 2) I could no longer register standard users (i.e. users that are just users, and don't want to submit an entry in the directory).

Thank you,
Vincent