What a great place to be in. Lots of work and a Thermo 1000 litre unit (£8-15k).
If you are serious about switching over to WFP with that many employees then forget the backpack/trolley route. (Option A)
Only go down the route of Option C if you have enough work that will always stay Trad, i.e, a lot of commercial inside work, otherwise just make the switch completely.
Both of your other two options are feasible, albeit with losing some of the advantage of a thermo pure system.
If you go for Option D then I would switch off the heating on the Thermo-Pure as it will be of no value. If you choose this route to run 3 vans from, then you may need another tank (IBC 1000litre £35) as a secondary storage. If you then have 3 vans with a 500 litre tank in each, you simply fill up each morning and go off to work whilst your system will replace the water in about 6-7 hours.
Option B would also be feasible and would have the advantage of utilising the Thermo-Pure in at least one of your vehicles. Decanting would take a bit of time - you would either need a 12v sub pump to stick in the thermo tank and pump across to your other tanks or rig up a multi fill system so that overnight the system filled all 3 tanks at once.
What I would do is the following: (only my opinion)
There are two real options when it comes to running multiple vehicles with WFP.
Option 1
Have a suitably large holding tank at base (either 1 or 2 IBC tanks £35-70) with a suitable RO system feeding them. A couple of 300gpd RO-Man units (£500) would do if you have good water pressure or one 4040 system (£650). Then have a Submersible pump and hose (£70) to fill all three of your vehicles tanks each morning. You could fit whatever size tanks you think would work in your vehicles. I would say a minimum of 500 litres for a two man domestic round team, 300-400 litres for a one man team and 800 litres for a two man commercial team. This is the cheapest and most reliable method of running multiple teams.
Option 2
Have a tank and overnight RO system in each vehicle. If you are on good water pressure then a simple 300gpd RO-Man system in each vehicle (£750) with shut off valves. Or a 4040 system in each van (£1950). This does give the option of the vans filling in separate places and even topping up if needed. The disadvantage is cost and a lack of a reservoir of water for quick refilling.
Using your Thermo-Pure.
You could fit the Thermo in with either system, Option 1 would give you the advantage of using the good 4040 base on the Thermo, all you would have to buy would be a 1000litre IBC and sub pump.
Fitting it into Option 2 would save you having to buy one of the RO systems and would mean that one van used the heating part.
I presume that the Thermo-Pure is second-hand (otherwise it would be rigged up in your van already by Ionics). If so, why not sell it and this would completely fund all of the parts (RO's, tanks, pumps, vehicle tanks etc) for either option above.
Just my thoughts having spent 7 years running a multiple vehicle set-up. I use Option 1.