Ouch, thats not cheap for the condition....so at 700 euro, does it at least have also paper work and a number plate?
Now, let me tell you a little story i read in "classic bike" magazine, some 40 years ago, when i was just starting, it is still true....
to restore a bike you need three elements
1. skills, 2. time, 3. money.
Sounds like you have a lot of time,
Not so sure about your money and technical skills, hope you have them too..
you can compensate a bit if you are lacking an element with the other two, but not completely
I am afraid that this bike will require A LOT of money and skills to put right and eventually,
it will be cheaper for you to buy a good running bike. in italy a decent XT600 is 1500 euro. You will easily spend another 700 just fixing yours...
I think this is a good bike to keep as a spares bike and learn how to work on it, but from the condition I see, it will be a very big and expensive effort to make it run right.
As for the needle, if its the one from the slide carburator (left), then i would leave it where it is now.
First, download the manual, all the specs (also for the carbuator) are there.
https://ln.sync.com/dl/3dcd06fc0/rwn.../1602417900014
Before you try to start the motor,
change oil and filter. dirty old oil will ruin the motor.
Good luck, you are going to need it with that bike