Starting by not expecting to rip every client off would be a unique selling point.
For instance, we all know Howdens gives the "trade" up to 90% off their hugely inflated customer prices so why does Mr Kitchen Fitter, who is already being paid his day rate, think it is fair game to rip off the client by pretending that he can get the £10k kitchen for £5k when in truth, he can buy it all day long for £1k ?
Adapt to suit your room of convenience but you get the gist of it.
You will get far better advice on software on kitchen-related forums.
Re supplies - it really does depend on what you are profiding - generic flat-pack, or specialist high-end? My friend is in the letter - he has most of his stuff built to order in Germany (it's f-ing heavy, too!)