Internal vs. External Storage for a new Macbook
Solution 1:
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.
Solution 2:
Well, besides if it is a comfortable solution, your intentions are possible.
Having a MacBook Pro with an SSD (because it is fast, consumes less power than a HDD drive) with the OS and applications, you can use the external HDD via (USB 3.0 and Thunderbolt) to store the application data, like ITunes Music. If you don't have it plugged in, e.g. while your are on a trip, you can't just simply access it.
I myself have an 512 GB SSD with a 128 GB SD JetDrive Lite (http://www.transcend-info.com/apple/jetdrivelite/). It is always plugged in, so always usuable. The 128 GB is used for music, pictures, books and documents of all kinds. Most of my work is stored there. Still easy to replace. I rather doubt than the SD card or SSD will worn out within the next 2 years.
For both solutions I have experienced no side effects or bugs so far.
Solution 3:
I also chose a minidrive to upgrade my storage. But i bought mine on TheMiniDrive and it works great. I think is the simplest solution and if you need more than 128GB they can provide the device. Ask through email. Mine is 256GB