I'm contemplating purchasing a DI2 frame and running mechanical on it. The seller states:

"ELECTRIC OR MECHANICAL SHIFTING FOR MECHANICAL SHIFTING YOU WILL NEED A CABLE GUIDE UNDER THE BB."

Is this factual? If so how labor intensive and skilled is the job at hand? If it does work will shifting performance suffer?

I think it is a $2 piece of plastic. Unless it is Campagnolo's version. If there is a bolt-hole, it is trivial. If not, proceed with caution (or glue).

It’s all about the cable tension!

https://framebuildersupply.com/produ...et-cable-guide

Or you get one for free off a scrap bike. Double sided tape if you don't want a hole. once it's on, the cable tension holds it.

Those plastic under the bottom bracket shell cable guides were included with every Shimano group. Check with any reasonably well established bike shop which should have a bunch of spares and you should get one at negligible cost. The attaching screw usually comes with it but, as noted, if your shell isn't drilled, double face tape or contact cement will hold it in place just fine.

Using rubber cement applied to one surface and pressed together while wet will hold sufficiently well and be easily removable. Or drill the hole and apply paint to inhibit rusting if steel. It might stay put by itself when the cables are under tension but securing will make things go easier.

IF you do drill a hole, which I have done, make sure the screw does not impinge on the BB housing or shell.
You need a screw no more than 3/8" long. As noted the plastic guides are supplied with all the Shimano
brifter sets I have ever gotten and I have several left over. Any LBS should have some laying around.