Since everyone's situation is different, this is a really tricky question to answer. Each city has it's own codes and guidelines. And the last thing you want is to finish installing lights on a house and have an unhappy customer.
That's a tricky question - it depends on what style of lighting your customers want.
I would not purchase massive amounts of anything in advance - you don't want to invest in a lot of product that you won't use.
Many installers use 1000 foot spools to use in conjunction with incandescent and LED bulbs. I would suggest that if you do that, you consult with a master electrician for instructions on how to build those. (you have to install switches and terminate them properly)
In the beginning cords with plugs already installed is a great way to start.
I'd show the customer samples of what they think they want and let them see them outside at night. Many times people think they want LED but change their mind. It's better not to have to take those items back and eat the material expenses.
Think about how long it took to decorate those 3 small homes and use those timeframes for your estimates. If you are installing in a commercial installation, I would plan to subcontract to a master electrician to work with you to make sure all building codes and electrical guidelines are looking good. Everyone seems to have a friend or brother-in-law who is an electrician.
Use those three houses as your model homes if appropriate. Use your own home as a starting place to show people nets, icicle lights, mini lights, C7 and C9 lights to give them ideas.
Our 1000 foot spools, installation clips and hooks and LED bulbs are all packaged at higher quantities for installers and are listed online at our distributor pricing.
I hope this helps! Let me know if you have any questions.