Yes the ground floor in English English is street level and first floor is one floor up.
Grammatically you're right, but because there is only one ground-level, it wouldn't make any sense to say "This ground-level". At least I cannot immagen any situation to say it so.
Can "földszint" be used in the sense of starting from the bottom? As in "we got in at the ground floor of this investment"?
I don't know for sure, but idioms in general don't translate, so I'd guess not.
My dictionary gives:
get in on the ground floor: részvényeket kibocsátáskor előnyösen megvásárol, előnyös helyzetet biztosít magának, az elsők között kapcsolódik be vmbe
I don't know what they say but they don't include the word 'földszint'.