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1) Carbonite has some really great members and not to mention even greater deals from time to time. So definitely join, if you know someone there that can vouch for you, get them to when you are looking to buy from someone.
2) A used SSD should be perfectly fine (go for under 2 years though - maybe 1 year old max) and just make sure to get a Hard Disk Sentinel report from the seller, that gives you a lot of info about the drive. Also ensure there is a warranty still (most of these are now held by Serial numbers so you don't even need the proof of purchase but proof of purchase is just better).
3) My recommendation is a Samsung 850/860 EVO, they come with 3 year warranty (so 2 if 1 year old) and they are highly rated drives. I am using the 256GB 850 EVO and..... I've had it for a year already, time flies! Well, it has Windows 10 installed, PUBG and League of Legends and I still have 156GB free space all the time, I haven't needed to put any other games on it. Check online if an SSD will improve a game because GTA V doesn't benefit much at all since the entire world is loaded during the loading screen, so likely VRAM or CPU since GTA V is CPU intensive.
4) The read/writes of an SSD that people used to worry about in the past aren't as much an issue for home use anymore. The average user won't get anywhere close to the maximum no of writes an SSD can take. There are articles out there to prove it.
Just ensure you backup any important data, a HDD or SSD can fail at any time no matter how healthy, over-worked or underworked it is.
PS: You will absolutely love the boot time of an SSD with Windows. Mine is 20 seconds to Desktop and launching programs with no issues (although programs on the HDD do tend to be slower to launch initially).
General rule of thumb:
SSD for Windows, Games (that can benefit and frequently used) and Programs that you use a lot that can benefit.
HDD for Documents, Music, Pictures, Videos, Non frequently used programs, large games that don't benefit from SSD.
