I wouldn't worry too much about the engine in the car, its not like you HAVE to use the engine. I'd concentrate on getting one with the right running gear and gearbox.
You can use any engine off the sierra and any gearbox. Its just that the pinto and a type 9 is the easiest to squeeze into a kitcar. In all honesty, the more you read about kitcars, you'll probably end up modifying your scope / design.
I decided to source my engine separately, because it will be a long wait for the perfect donor, plus I wanted a pre-cat engine as I think its easier to set up a carb engine than fuel injection / cat converter.