The big enemy you have is damp, all the rest is pretty small beer, but damp is a real killer in a workshop as I found out last winter (it has been known to rain in Wales!) when I went through a similar exercise. It looks as if most of the trouble is coming under the plinth that the wall panels sit on, plus a bit through the slinging holes and I'd tackle those issues head on.
Without seeing more detailed pics, my vote would be for the DPM and concrete raft on the inside floor; it's fast, easy and relatively cheap. Put a course of blue bricks across the thresholds of both doors, and lay the concrete making sure the PVC is turned up above the new floor level. Then make a ramp for the main door entrance to get heavy stuff in and out; you can leave the side door with a step if you like. On the outside, thoroughly jet wash the path / wall junction, and using a PVA mix plus some PVA painted on the interfaces to ensure adhesion of your cement, lay a fillet. If you want a flexible finish, don't bother with the fillet, just give it a couple of thick coats of bitumen.
I wouldn't hand mix, that's a tedious pain; half a load of ready mix and you could have the whole job done in a day and a half. Day one, jet wash inside and out, lay the blues, drop the PVC in together with any reinforce. Day two pour the concrete, lay the fillet or bitumen, bodge the ramp. Four hundred quid should cover it easy, and you won't have any b***** digging out or rubble to get rid of, the path can stay put.
Again without seeing the penetration through the gaps / corners, a can of expanding foam works wonders.