I'd say 40mm is too much generally anyway.
With no load, -20mm rear is max achievable for geometry. Add load to that, and your over with no self levelling. It's not instant disaster, but inner edge wear gets progressively worse accordingly from the -20mm it's set up to. And it will need set up.
You could fit the -40mm kit, but needs a couple of extra insulation pads to raise it back up "IF" there's enough of the centre peg left to locate the spring. Too many pads and the spring could POTENTIALLY fall out as the pegs are relatively shorter for the spring to sit on. I know -30 springs are ok with a 10mm extra pad arrangement, as I've fitted them previously, but do remember thinking -40 would be a bit dodgy spacing out with that many pads iirc.
So, if lowering I'd say no more than 30mm drop, but add 10mm worth of insulation pads to get the rear back up to no more than 20mm drop. But if you carry loads, which is arguably the point of an estate, then stick with sl. If you get trade club access they are, or where, £150 odd. Half the retail price...!
Still not massively cheap, but cheaper than a whole set of springs and full geo set up.