G'day D,
What subaru do you have? Only asking so we know what you're working with.
The EJ25 would probably need a loom change to go with it. A mate of mine has an SF forester and dropped an EJ22 in where the EJ20 used to be using the factory wiring and ECU. He just used the same timing sprockets and intake manifold from the EJ20.
With any engine swap - regardless if it's a conversion or not - it's always a good idea to do the front seals, rear main seal, timing kit including a new waterpump and HGs if they need to be done. The coolant crossover O rings could be a good idea for piece of mind.
With any engine from a PAP yard, you've got to be cautious as to why the vehicle is there in the first place. Is it there due to a dud engine or a running issue? Is there obvious damage that's written the car off? You could be buying someone else's problem - and if you're not handy on the tools to DIY you could be in for a lot more money in the long run You also have to be willing to work on it to see what the issue is to fix that before it ends up in your vehicle.
Cheers
Bennie