Welcome
Your Mars II 400W is a great light, I think it draws actual around 200W...
And the GO Box will serve you well.
A 2' x 2' x 5' tent would be perfect...the light could probably sufficiently
light a little bigger area except for the fact that it's square in shape so
a square floor plan on your tent is a good call, IMHO.
For strains...you say you want purps/color?
Head straight for Sweet Seeds Red Family. Then pick the one that sounds
most intriguing to you
Soil...6 months ago, I would have said Fox Farms Happy Frog, but so many
issues regarding the Fox Farms quality control on their soils. I have made the
switch to Roots Organic Formula 707 myself and will start my first 4 or 5 plants
in it in the next couple days.
Poke around read some stuff and start getting prepared, It's gonna be fun!
Items to research:
pH measuring devices (solution testers and soil probes)
The use of Garden Lime (dolomite lime)
The need for more CaMg with LEDs
Myco soil amendments
Different pot/container types (airpots, smartpots, etc.)
Air exchange for the tent (intake/exhaust)
Odor Control
Feeding to runoff
Oxygenating your water/dechlorinating tap water
That should get yah started

And again, WELCOME!