If he's replacing the SWCM (your controller), then he'll need the MFSW airbag loom too, but I guess he knows this.
I'm pretty sure a member on another forum had a lowline with MFSW - problem being he couldn't change track with the MFSW, but that was with an RCD300, and I'm sure it worked otherwise.
He will definitely benefit from a cluster upgrade, even to midline, as he will now have trip functions available with a highline SWCM.
Where is he and what's the age/details of his car? I have my diesel midline clocks going and I could fit them if he was in the NE. Others on the forum could do the same I'm sure if more local to him.
Certainly to get the most out of the whole setup, it's best having at least midline clocks.