Get bot events.
When you click which boss you want (assuming this is what will happen), use the event onActivate>Player>editAppearance, and edit the appearance to your liking.
Here's an easier way to do it.
1. Download
Bot Events (by Amade).
2. Create a single player server.
3. Place a vehicle spawn.
4. Spawn a bot.
5. Change your avatar to what you want the first type of boss to look like.
6. Say, "/applyAV".
7. Re-spawn the bot.
8. Wrench the spawn and click events.
9. Use the excact same events on the "button" that, when clicked, will make the player a certain type of boss, except instead of onBotSpawn, use onActivate and Player instead of Bot.
10. Do steps 5-9 over again for each button that, when clicked, makes the player a certain type of boss.
