Internal vs. External Storage for a new Macbook
It all comes down to your convenience.

The MacBook Pro (MBP) uses a proprietary interface for their SSD; PCIe 2.0 x4.  This is why they are so expensive as you have noticed.

Upgrading the SSD in the MBP to a larger SSD, whether you get it at time of purchase or you get it later on, is going to be an expensive proposition and the only reason that you do this is because you want as many of your files with you at all times without having to connect a portable drive.  That's the convenience factor.

However, if not having all your files readily available on your MBP and toting around an external drive is not a problem, then going with a smaller SSD and an external drive is definitely a good choice.

As for what you want to do, there is no problem with keeping your files like photos, movies, music, etc. on an external drive.  As for performance, you probably won't notice much of a performance hit depending on exactly what you are doing.

One caveat... You will need at least 2 external drives.

Why?

You are going to back up your stuff, right?  If you are going to be toting around another drive, there is a greater possibility of data loss; physical damage, stolen, or just leaving it on a table somewhere in a coffee shop.  You are going to want this backed up.  A USB 3.0 drive with sufficient space should be more than adequate to do a Time Machine backup during the night (where performance isn't going to hamper anything).

To know how big of a backup drive to get, just make sure it is larger than the total of your internal SSD and external drive.  If you have a 128GB SSD and a 1TB external, get at least 1.5TB.  If you are closer to 2TB of internal and external combined, get a 4TB.  If more than that, you probably want to look at some pro level solutions.

Ultimately, the choice is up to you whether you want the convenient method of having all storage internal to the MBP or you don't mind having to plug in an external drive.