you really might want to try the soil that you mentioned ...i have seen and heard of ppl buying a cheap top soil or digging some up in the yard dumping it in with a sand cap or gravel or w/e and haveing no problems ..not really talking about the walstad method of which i know very little about.
or you could go to home depot and buy the cheapest top soil you can find lol ..its like $1.50 for a 40 pound bag...and mineralize it by wetting and drying it out on a tarp/trash bags and rinsing it (theres a thread probly in substrate on how to do it from Aaron Talbot i think).
I can say that it has worked for me(so far) ..i just finished mineralizing it after about 10-14 days and the few plants that i have in there now are doing amazing under low light..i can see the growth each day. its a 55gal with 54watts T5 HO . you should look into soil..it might be for you ..plus its super cheap to do. for me $14 for potash and dolomite (lots left over too), $3 for two bags top soil, 50 lbs pool filter sand $6 ...this covered the 55 gal 1-2 inches before the sand.