Recovering data from flash that has been overwritten is expensive enough to require the resources of a government, which of course considers money to be free. So you really only need to focus on the free space--"deleted" files there are easily recovered because they aren't truly deleted at all but the space they take merely marked as available.
You can use Windows' own built in cipher command like so:
to zero out this space on the C:\ drive.
Alternately, you could download sdelete
from Microsoft, which gives you two options:
sdelete -c C: /accepteula
which cleans free space to military DoD standards--by filling it with random data which is harder to recover from but obviously no good for garbage collection and TRIM
sdelete -z C: /accepteula
which fills with zeroes only to improve SSD performance too
In each case you're only adding up to four P/E write cycles to the blocks holding free space, hardly anything to be concerned about.