Out of the 2 choices the OP has - I think the Merc is the "nicer" car (nicer interior, ICE, looks etc), however the Touareg is the "better" car (stronger engine, better built, more capable offroad etc).

If I was spending £25k on a daily driver, which needed to be a quality, robust and reliable tool, which could handle the odd building site, I'd probably look at Hyundai (Santa Fe, Tuscon etc.) or Kia (Sportage, Sorrento etc.). You can easily pick up a 2018/2019 model from a dealer, with the remainder of the 5/7 year warranty within your budget. Badge snobbery aside, they're very decent cars.