2/3
OF 1/3 Perlite would give you great drainage. I'd say that ratio is optimal, just remember by sacrificing some of that media you can start feeding earlier, but not too much earlier - still want to give the plants 18-24 days with no nutes, just plain PH'd water.
Just two stacked 18 gal rubber maids gives you enough for probably 4 1GAL grow bags, BUT your lighting doesn't power 4 plants.. I'd say two plants at the most. I'd go with bags over buckets or pails or head to the dollar store and find a rectangle shaped bathroom trash can. You can fit two 2GAL grow bags in that space with enough room for some branching. That's probably your best bet if you don't go with the trash cans.
CFL lighting doesn't put off too much heat although it puts off some. Also if you haven't purchased the PC fans yet, just get a 4" Holesaw bit for a drill and use this guy -
http://www.newegg.com/Product/Produ...-+Home+Automation-_-Suncourt-_-9SIA05W0194941 I used a 6" version 125CFM for my CFL cab that was about twice the size and it was adequate exhaust with almost 250W of CFL lighting.
Is smell a concern for you at all? With two small plants you can probably get away rigging up an ONA diffuser or something of the sort.
EDIT:
After looking up the 18 gallon dimensions;
Measures 24" x 16" x 16.5" inches
You may want to opt for smaller pots just to limit the size of the plants - I say this because 37" of vertical space isn't that much. Especially after you add in lighting and the height of the pots. Maybe use 2 gallon grow bags and fill them up 75% of the way and roll the remainder of the bag down or cut it off. Although 10" (standard 2 gal bag) with 18-24" plants would still leave you enough room for the lights depending on the size of the fixture.. hard to say on that one with all things considered, just throwing this opinion out there.
