63 yr. old "dreamer" needs expert advice

just another "indie" dreamer buying fully loaded 27' iMac with plans to edit feature length (non "special effects" types) films/tv productions.
plan to purchase 16Ram memory separately to save $$.
is this adviseable?
will it void warranty -- how could they tell?
suggested place to buy?
also..,
which drive choice would you recommend?
a. 1 TB Serial ATA Drive
b. 2 TB ATA Drive [$135]
c. 256GB Solid State Drive [$450]
d. 1TB Serial ATA Drive + 256 Solid State Drive [$540]
e. 2TB Serial ATA Drive + 256 Solid State Drive [$675]
with 2 thunderbolt drives is it better to upgrade drives externally or is 256GB ssd best choice for speedy internal operation?
$$ is important but would prefer not to get off on wrong footing.

THANKS.