Anyone on here know if there is a way to make a model spawn into the map when the player buys a door?

You will probably have to script the entire door yourself.
Personally I would already have placed the model on the map, and tell it to move into place.

Tutorial Here:
https://confluence.ugx-mods.com/display/UGXMODS/BO3+%7C+Purchase+Loops
Code API is in the black ops 3 directory under "docs_modtools>bo3_scriptapifunctions.htm"

Benefit to this method:
instead of "Press [E] to Open Door"
You can edit it to say "Press [E] to Open Power Room" Or something.
You can do literally anything if you can code it. I custom code all of my doors.

Downside?
Time Consumption

DoorTrigger = GetEntArray("zombie_door","targetname");

foreach(Door in DoorTrigger)
{
if(Door.ModelSpawnDoor == "Yes")
{
SpawnModel();
}
}

This is basic psuedo code, add a custom KVP called ModelSpawnDoor and give it the Value Yes.

Add me on Steam if you want more help, I might make a tutorial on model spawning actually.

DoorTrigger = GetEntArray("zombie_door","targetname");

foreach(Door in DoorTrigger)
{
if(Door.ModelSpawnDoor == "Yes")
{
SpawnModel();
}
}

.

^ That method is easier. Good suggestion.

http://steamcommunity.com/sharedfiles/filedetails/?id=834085206

Here you go

I added you on steam bro