I must keep my 07 S-type outside and use it as my daily driver. Are there any disadvantages to getting it rustproofed by say Ziebart? I'd greatly appreciate any information about this.

 

Will they take the plastic underpanels off? I don't just mean the engine one but ALL the panels.

I'd DIY - and take those panels off while doing it. Cover anything that doesn't / may not like the solvent etc (plastic bags / cling film).