It sounds as if the existing state of the wall is a prime consideration here. I'd be inclined to get a professional round to have a look. If it's fairly sound and you already have a suitable piece of timber for a lintel and are prepared to get rid of the stone yourself, it probably wouldn't take a pro more than 1-2 days. I know a builder who'd be good for the project if you decide to follow that route.
Brian
Brian