guessing from the time of your post and the used terminology (“spanner”) I assume you’re located in the UK.
Series (I-III) Jags are comparatively easy to handle both in terms of mechanics and electrics. You should be aware that they - and many parts - are big and heavy. So a spacious garage and/or workshop are definitely more of an issue than with a Spitfire or MGB.
Apart from that rust will be your prime issue. If you like the SIII body style there is one more rust trap around the front and rear screens as they are glued in and the decorative rubbers keeps water inside and causes extensive and often terminal rust damage at the A- and C pillar.
The rest is fairly normal: rusty front and rear valances as well as rear quarter panels can be repaired cheaply. Front wing rust is typical, but can be remedied as these parts are bolted on. Outer and inner sills are bigger issues. For checking cars it’s very helpful to loosen the two screws fixing the rear seat to the floor and look underneath. If this part of the floor pan that secures the rear axle is sound, the rest will be doable.
Even with technical skills I’d always go for a possibly original car with limited mileage and in a reasonable state of care, as you’ll find that you’ll be able to keep alive most of the original parts with a nice patina and it is a lot more easy to know your starting point for diagnostic work.
If you’re into a really nice project you might think about getting a rust-free hull from the southern US and transplant a good original engine and interior from a rusted-out low-mileage UK car.
There is a comprehensive buyer’s guide available on this site http://www.jag-lovers.org/xjlovers/xjbuyersguide.html
75 XJ6L 4.2 auto (UK spec)