I'll weigh in here - I have a 125g heavily planted dirt tank with a sand cap. I would wet the soil now and let it soak until it's time to fill. You'll want to drain it so that it's not completely liquid, and doesn't just move when you dump the sand in. A huge portion of people with dirted tanks use sand, so dont worry about plant growth. I have a full carpet in my 125 - half is dwarf baby tears and half is dwarf hairgrass - they both spread quickly and are very lush and healthy.
Sounds like you have the right idea. Sand is very a popular and successful cap. It is easier to plant in and fish seem to prefer it. Watch your parameters for the first few weeks - you're likely to have an ammonia spike with the decaying organics in the soil. Also, malaysian trumpet snails are your friend and will help turn your sand and avoid anaerobic buildups.