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help with your first question, but a few points to consider below.
On a 245/35 tyre, the sidewall will be slightly higher than on a 235/35 tyre, so depending on brand of tyre, you might experience a little more flex in the sidewall under cornering. The larger overall rolling radius would also mean a very small speedo error reading. There’s likely to be a price difference between a 245/35 and 235/35 tyre, but the wider/higher tyre won’t necessarily be more expensive than the factory tyres.
Fitting different sized tyres to factory spec tyres is also considered as a modification for insurance purposes in the UK, so you’d need to inform your insurer.