any issues when installing an SSD on an iMac?
Yes. OWC sell fantastic SSDs that are fully compatible with Macs.
Your iMac only has SATA 2, not SATA 3 - so you are limited to a speed of 3Gb/s. Bear that in mind when reviewing advertised SSD speeds. 6Gb/s is not possible on your iMac.
I would recommend one of their 'Mercury 3G' drives since buying a 'Mercury 6G' drive (ie. SATA 3) is a waste of money for your current iMac...but you could of course reuse it in the future.
To be clear, a SATA 3 drive will work in your iMac - it'll just negotiate a speed of 3Gb/s instead of 6Gb/s.
Has OWC SSDs that are equivalent to the Samsung 850evo as far as specs (speed etc.) are concerned?
Yes. OWC sell fantastic SSDs that are fully compatible with Macs.
Your iMac only has SATA 2, not SATA 3 - so you are limited to a speed of 3Gb/s. Bear that in mind when reviewing advertised SSD speeds. 6Gb/s is not possible on your iMac.
I would recommend one of their 'Mercury 3G' drives since buying a 'Mercury 6G' drive (ie. SATA 3) is a waste of money for your current iMac...but you could of course reuse it in the future.
To be clear, a SATA 3 drive will work in your iMac - it'll just negotiate a speed of 3Gb/s instead of 6Gb/s.