This is exactly what I did in 2015 when it was time for a new machine and I was not happy with the offerings in the iMac line at that time. I bought a 15" 2015 MBP with i7, 16 GB RAM and 512 GB SSD, and supplemented that with (at that time) the Samsung T1 external SSD, and over the years moved to T3 and then T5 external SSDs. This has worked very well for me! I still have that machine, which unfortunately right now is at one of Apple's repair depots getting a top case/battery transplant as a result of Apple's recent recall. She served me very well as my workhorse for three and a half years and I don't intend to part with her any time soon, as now she comes in handy for various things and is frequently pressed into service for an archiving project I'm working on. Just mentioning all this to say that yes, a 15" with i7, 16 GB RAM and 512 GB SSD can work out quite nicely with external SSDs to provide extra storage space.
That said, though, this past December I did purchase a 2018 15" MBP with i9, 32 GB RAM , Vega 20 and 1 TB SSD, and unsurprisingly, yes, it's a bit faster than the 2015 one and gives me the opportunity to use Samsung's X5 Thunderbolt 3 external SSDs now and in the future, too, which is great when one needs even more speed than the USB 3-based T5 SSDs. I don't use this for video, but I think that is why many purchasers would. I still continue to use my T5 external SSDs for most things, though, and they have been very useful for swapping data back-and-forth between machines. For travel they are ideal, since they are so small and lightweight. They come with two cables so it is simply a matter of switching cables when using one machine or the other. No dongles needed.....
For a while I also had a 13" MBP and I agree with others here that really, for photography, unless you are going to purchase and use a separate monitor with it, the 13" just does not provide quite the same amount of screen real estate that is needed and desirable when editing images, especially if doing them for paying clients. I use my 15" MBP for editing my images but they are only shared on the web and I only do this for my own pleasure as a hobby, I am not a professional photographer. I definitely would go with a 15" MBP! This offers the flexibility of being able to edit (say, on location) without an external monitor or to edit at home using an external monitor. Since this is for a business when tax time comes you will also be able to write off some of the expenses related to the purchase of the computer itself plus any additional equipment, too, of course.
Right now, I think it would make more sense to purchase a current model rather than looking back to 2015, especially since this IS for business. A current machine will serve you well into the next several years, and there should be no issues down the road for at least five or six years in terms of compatibility with whatever Apple's MacOS looks like at that time. Apparently the keyboard issues have been resolved for the 2018 and 2019 MBPs -- I haven't seen a barrage of complaints and so far I have not experienced any problems with the keyboard on my 2018 MBP. I also have had no issues with a need for dongles and adapters, as my printer is wireless and my external peripherals are also either wireless (BT) or have the USB-C connector on one end (including my SD card reader). My older external HDDs are not a problem, either -- I simply purchased the appropriate cables for them, with their native connector on one end and a USB-C connector on the other. I now can use them with either of the 15" MBPs as well as with my 12" MacBook. That said, I do have a couple of small, tiny little adapters that I keep in my travel kit for those occasions when I might need them; so far that has happened only one time.
Good luck with your decision and purchase! Let us know what you decide and how things work out.....