Hi Paxil,
The lowest safe height is about 700 ft in my experience but you might want to make it nearer 900ft.
The height is above the field (Not sea level) so if the field is at 1000ft you need to be at least 1700 - 1800 ft for a safe drop.
It takes 10 troops to caputure a field (Max load of C47 & M3), but they don't all have to have dropped from the same plane/vehicle. i.e 1 M3 & 1 C47 could lose upto half of there troops each and sill get the field, (It never ceases to amaze me that missions frequently have 2 of more buffs, half a dozen fighters and 1 goon... oh well).
Hope this helps
TTFN
snafu