Don't confuse a square wheel setup with a square tyre setup. You can run the former with staggered tyres and have no issues at all. Unless you're aiming at a specific track only setup AND have a significant ability behind the wheel, I would strongly recommend against running a square tyre setup. You'll be promoting snap oversteer and an incredibly twitchy car, which can suit some drivers but only in specific circumstances.
If you want to tame understeer, do it by adjusting the tyre pressures and the geometry. You'll still get dramatic results if required, but it's much easier to tweak as you need to. If you're staying with 18s (and I would too) then in the wheel sizes you've mentioned I'd move to a 245F and 265R setup, that will still fit but won't upset the balance of the car.
If you did run a square tyre setup too, you'll pretty much need to drive with VDC off all the time otherwise it'll be incredibly and potentially dangerously intrusive.