I wouldn't sweat the substrate too much, especially since you will probably be doing water column fertilization. Go with something you like that is in your budget. Undoubtedly, the ADA substrates are probably the best, but in a tank that size they will get rather pricey.
Laterite is a type of clay, whether it's volcanic or not, I don't know.
pretty much any porus baked clay is going to have dissolved minerals and act as an ion exchange site. You want baked so that it doesn't turn into soft mush once you add water. You can even use crushed terra cotta.
Look into the "aquariumplants.com own" substrate. It costs $45 for enough to give 3 inch coverage in a 4X12X18 (55G). So far it is working very well for me with very little compaction after 6 months. If you are thinking about swords and such then you don't have to worry about the acidic soils for black water biotopes.