Who owns the lot? If the builder owns the lot, then you don't need a realtor to buy the land, because typically the land and house will be bought together form the builder.
When we did this a few years ago, we bought a lot (using a realtor), and then contracted with a builder to build one of their plans on the lot we owned.
Now, I would be VERY VERY careful to make sure that the plan as shown by the build is EXACTLY what you want. Often it's one of their spec houses with 'standard" features. If you want to make changes to the plan, as much as possible you should make a detailed list of what you want to change and how much that will increase the cost. For example, when we did this, we took the builder's spec house, and by the time we added all the changes and upgrades we wanted, it was an additional $150K. The original house spec was well under our budget, so we could do that. Things you should consider.....
cabinet upgrades
flooring upgrades
counter top upgrades
bumped out the garage to make it 3 vs. 2 car
added a mudroom/walk in pantry
straightened an odd angled wall, which meant a longer beam was needed (that was a $5K upgrade!)
made the basement ceiling 9 feet (so it could be 8 feet when finished)
made all first floor ceilings 10 feet
made second floor ceilings 9 feet
upgraded the siding
upgraded the roofing material
added a TON of outlets
added a TON of overhead canister lights
added in wall conduit for TV and cable/satellite wires
added pre-wiring for ceiling fans in all bedrooms
high outlets on walls for TVs
upgraded paint (plan came standard with one color, we ended up hiring our own painter to come in at the end to re-paint because it was a lot cheaper)
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Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)