I think that generally they are pretty solid however there appears to be a weak spot on the oil cooler (seach for "Mustang oil cooler failure") that CAN lead to catastrophic failure (oil / coolant mixture). If I were buying a 5.0 GT then I'd factor in about £1K for a replacement.
If you find a 2017 model that you like make sure it is registered AFTER April 1st, 2017 models registered before April have a RFL of £695 a year, registered after April 1st the RFL is £180ish