The concrete floor will need to be cut, the new toilet and drains will be about 12 feet from my "main line" which I going to TEE into. Will this need to be vented? I was told today by a plumber that at 12 feet I "will not" need a vent pipe!! Is this correct?

Massillon, your plumber did not advise you correctly. For 12' run, you will need to vent your toilet with 2" vent pipe. You can connect this new vent with existing vents about 48" above the floor.

You can connect this new vent with existing vents about 48" above the floor. Providing it's a dry vent with nothing draining into it. Otherwise you must run out the roof or revent back into a dry vent at least 6" over the top of the flood rim of the fixture you're reventing back to.
Why not go all the way and install a bathroom group while you have the cement tore up?
Most bathroom groups are roughed in like this.
Toilet connects to sewer main or the stack vent. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof or revents back into a dry vent in the attic.. The toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes and also The Standard Plumbing Code Book in 90 percent of the country. Check your local codes to make sure you're not in the excluded 10 percent.. The vent off the lavatory may be run out the roof or revented back into a dry vent in the attic or if you're reventing back into a fixtures dry vent you must make your connection at least 6 inches over that fixtures flood rim. (see image)
Good luck on your project. Tom

If you work under UPC - came back to talk to me... Otherwise, Tom is very correct in his answer...