I would consider buying that Jag but I'd take the view that it's a bit of a lottery. You could go years without the need for many repairs or it could be a can of worms. Take along someone who knows that model to check it out throroughly.
You said: "It's apparently in excellent condition (a few minor rust patches - one on bonnet, couple of wheel arches)". This seems unlikely in my experience. Rust is like an iceberg.
Together with rust, the air con system is another aspect which could be expensive to put right, if you need cool air.
Often-quoted advice is to spend more on a good car in the first place. It's frequently cheaper than a low cost purchase plus repairs/restoration.
In the handbook to the car, Jaguar would have strongly advised against using unleaded fuel under any circumstances, at the risk of causing serious engine damage. It's stange then that most people do now use unleaded fuel. The lack of lead isn't really the issue. It's the need to maintain the octane rating, or adjust the timing (probably already done) to allow the use of lower-octane fuel.
Good luck.
Regards George