GTX 970 SLI / upgrade

Hi,

mother board - Gigabyte GA-Z97X Gaming 7 10 Gb/s M.2
processor - i7-4790K @ 4GHz
power - Corsair RM 750
RAM - Corsair 16 GB (2x8 GB @ 2133 MHz)
video card - Gigabyte GTX 970 G1 Gaming
https://www.gigabyte.com/Graphics-Card/GV-N970G1-GAMING-4GD-rev-10-11#ov
monitor - Philips 272G5 G-SYNC 144Hz

Q1. I build my PC around 3 years ago (purely for gaming) and it is still going strong. When I put it together I planned to add in future another 16GB RAM and most importantly second GTX 970 G1 in SLI. I want to do this now however I realized that I can only buy this card second hand. I would prefer to buy brand new card that is compatible with the one I already have. Can you advise me please which card should I be looking at?

Q2. If I were to upgrade to two brand new cards which should I go for? Bear in mind my current spec as I do not want to upgrade power supply or spend a fortune.

Q3. Should I consider adding SSD and if yes why/which?

Thank you for helping out an amateur.