My advice: don't pre-sell anything unless a) it's simply to get a public beta in the hands of customers that want/need to use it right away, and b) even then, have a practical and do-able timeline in place for finishing any documentation, etc. that needs to be done for the final, polished product. And, above all else,
do not spend the money - just put it in an escrow account or something similar.
I've been involved as a consultant with a company that took deposit money from customers, used the money to fund development of the software, and didn't deliver. It's surprisingly easy to screw up, so don't put yourself in that position. It's extremely stressful for the principals in the company, as well as the engineers tasked with the herculean task of trying to create something lasting while under the gun.