Based on the information you provided in your post, it is hard to answer your questions but I will address them in general terms...
Builder tie-in could mean a couple of things depending on how "raw" the land is. It most likely means that the builder must tie in to sewer and/or water lines.
Unless otherwise specified in the listing, you should be able to use any builder you choose. Be very careful, however!!!
You asked if there is a cost savings. If you are referring to the cost of building over buying an existing home, there can be a cost savings but you really won't know unless you compare apples for apples.
If you are referring to using the builder who is selling the lot, as opposed to using your own builder, that is really going to depend on what each builder is providing for the price. You will want to compare builders' packages (specifications), paying particular attention to the quality of the materials and allowances for things like cabinetry, lighting, appliances etc. These details can really add up if not included in your "base" price.
It's a lot of fun building, though! I've done it before and would love to do it again!!!! Good luck!