Excessive writes to flash storage wears it out faster. Particularly on a low capacity SSD (with less flash to spread the write load across and fewer reserve sectors to replace failed sectors), this could lead to accelerated drive failure.
If you're worried about hardware longevity or seeking performance: You're best off just using your HDD. HDD's can handle high write loads much better than SSD's can, and even the slowest hard drives still seek much faster than needed for a DVR workload (even the earliest hard drives from the 1950's took only about half a second to seek, which is probably good enough...). If you're worried about the additional spin-ups causing your 500GB HDD to fail earlier, and losing the data on it, consider buying another 500GB HDD and using a RAID configuration.
If you don't want to use your HDD to buffer to avoid noise, power consumption, or vibration: If you're transcoding to h.264 anyway (which doesn't take up much space), you might just want to use a RAM disk. Assuming a 6mbit/s transcode, you would need about 2.7GB of storage per hour of buffer. If your system runs Linux, you can use tmpfs which will also take care of swapping out data to disk if your RAM disk runs out of space. Here's a writeup of some steps that may or may not work well for this: https://www.howtoforge.com/storing-files-directories-in-memory-with-tmpfs (I haven't attempted this myself so I have no idea if it would actually work, but I would try it before I used an SSD)
