OK the reason for checking was around this time the support for SATA III in the main drive and optical drive bay were changing. You should have SATA III (6 Gbps) in the main bay and SATA II (3 Gbps) in the optical bay.
https://eshop.macsales.com/item/OWC/...GB/#comp_notes
You also do not have USB 3 in that computer [bzzt actually you do, see correction in a later post] and no thunderbolt [bzzt I'm wrong again, see later post], and one Firewire 800--but Firewire is pretty dead and you can't get any decent Firewire SSD drives. So all that says make most use of internal SATA SSD.... or upgrade the laptop. And I would try to stop using any external drives if possible, much easier to deal with and more reliable especially if moving the system between locations.
So you can add a SATA III SSD in the main drive bay, that is a no-brainer.
The Samsung 860 Evo is a fine drive to go with. Normal users are unlikely to notice not having the Pro version. And the Evo (and more expensive Pro) are available in 2GB and 4GB sizes at compelling prices. So for example it's possible for many people to stuff their VI samples on the boot/system drive.
I (and others) can't guess what you are doing, how complex your sessions are, etc, or how much total disk space you need. Disk cache can cover lots of sins, but your system also does not have much DRAM capacity, hopefully you have upgraded to 16GB already. If your sessions are small then setting disk cache to a few GB should be a big help, disk cache == normal is disabled.
If you need to you can add a similar SSD into the optical bay, an it will run at SATA II speeds, but you don't loose as much practical performance there as implied by the 2x raw bandwidth difference between SATA III and SATA II. and if you still need an optical drive move it to an external caddy (or just buy a new Apple external USB Superdrive) Especially if you just need the drive space I'd add this second drive--it may be a toss up as to wether it's better to put samples on the boot drive and sessions on the opticl bay drive or samples on the optical bay drive and sessions on the boot drive -- depends if your VIs stream or pre-load samples, how much disk cache might help you etc.. But since I see the optical driver as a waste of space I'd likely swap it with an SSD anyhow.
If you want to use the optical bay look for a suitable OWC tray adapter, but I would use Samsung 860 Evo or Pro SSDs, not OWC SSDs. If you don't have 16GB RAM already installed do that when you upgrade the drive(s). OWC RAM SO-DIMMs are fine, as are any mainstream vendor Viking, Micron , etc. just make sure they are speced for this exact model.