Soror SummerChild,
Yes you can buy a lot that is already purchased in a subdivision. That is usually done anyway especially in high end subdivisions ($190,000 $600,000) If you do not have to buy the lot separate builders add it in the cost of the house anyway, you just never know the difference, thats in the lower ($70,000 to $180,000). Some body has to pay for the lot either the homebuyer or the subdivison builder. Now, a contractor license, gives you teh license to build houses. You do have to take a state exam like a realtor or doctor or barber etc....
Getting a builder to save on cost....yes actually you can save significantly....this is also depending on the type of house, area, lot cost, aminities in the house etc. example on a $130,000 hous you see in a subdivision, a builder may be able to build it for $100,000. See it is all in in the builders sub contractors bids...meaning the roofers, framers, electricians, plumbers he or she chooses to bid on that part of building the house. Then they mark up the profit on the house after figuring the specs on the house. So if you had a builder/contrator you hired and gave him the house plans, you can hire your own subs to bid on the work and take the price tha that suits your budget (saving on cost) and you pick your own flooring etc. this way you tell the contractor yoru budget accordingly tothe type of house.
So yes you can save a lot of $$$$ just keep in mind the area of the lot, sub-contractors to do the work and not to mention YOU are over the project and can hire and fire a contractor that is not up to par likel, cutting conrners etc. Every penny counts. Now also keep in mind some people don't want the headache, so they just finda subdivision that is bulding and pick a lot and pick certain houses in the assortment "they" have picked out you can pick from versus you pick your own house from house plans from a architect or blue print wholesaler.
Both are possible, question is do you feel like it will be a headache? Hope I helped. If you need futher answers just let me know.