So far the advice to buy 3rd party RAM and storage is good if your goal is to save money Other don't mind paying more have convenient one-stop shopping.
One thing I would add if this is for photography is to plan a BACKUP system. With Apple using Time machine is your first line backup. You don't need a fast SSD for this because it happens in the background and you never have to wait for it. But the Time Machine drive needs to be LARGE. At least twice the size of all the data on the computer, 3X might be better. The reason is that it holds all the old versions of your data.
Also, it is possible to use two (or more) Time Machine drives and the system will Switch and write to the other disk each time This makes Time Machine very robust but still you need offsite. The #1 reason for data loss is theft of the equipment.
If your business depends on this data you need a lot of redundancy but if it is just personal use then a good rule of thumb is to always have the data copied to three different physical devices and at two different geographical locations. Business data needs a bit more safety than that.
For the local Time Machine data, a Thunderbolt connects hard disk works. If you want two then buy a drive enclosure that has room for two drives and configures then as independent drives. Low power "green", slow and large disk drives work well for Time Machine.
What I do is every few yers I buy a new Time Machine drive. I get the bigget disk that is affordabley priced at the time and then I retire the oldest Time Machine disk. Disks have a finite service life (five years max) and it is best to replace your backup drive BEFORE it fails.