I just started up my first iwagumi tank last night! It's alot of fun collecting everything and planning to get it set up, you'll enjoy it!
As far as lighting, with most carpeting plants, you're going to want to be in the medium-high or high light category. For a 150 gallon, I would definitely say you'd need a T5 set up. From what I've heard (I can't personally vouch for this from personal experience), the WPG rule has become obsolete with new bulbs, etc. I've heard others say that it was only truly useful when considering T8 bulbs. For my 55 gallon, I run 2 T5HOs.... so perhaps for your 150, you'd want to go with a 4 T5 set up?
For carpeting plants and iwagumi styled idea, you will definitely want to go with a CO2 set up. I run a DIY CO2 set up on my 55 gallon, with 2 2L bottles. I maintain right at or above 30ppm CO2. I do NOT think DIY CO2 would work for your 150 gallon.... you could probably make it work, but the amount of sugar and yeast you'd use, not to mention trying to organize all of the 2L bottles, would be a nightmare and more costly in the long run. You'd want to go with an injected CO2 system. You can do a DIY injected paintball system or a full out CO2 system. Unfortunately, I'm not the one to help you with this, as I have no experience with injected CO2 systems, and really don't know much about setting them up. Doing the learning for that myself now
As far as substrate goes, there are so many alternatives, it is hard to list. Aquasoil is regarded pretty highly, but as you said, can be very expensive. Flourite is another popular substrate. If you're wanting to have a nutrient rich substrate, maintain that fine grained substrate look, whilst not spending $100s just on your substrate, then you may want to consider something similar to what I, and many others, have done. I use soil as a bottom layer, and top it with a fine grained gravel [or in my most recent project, Safe-T-Sorb (a finely ground fired clay mixture)].
There is a plethora of information around here on the forum, so just start reading!