Definitly don't quit your job. This is a good opportunity for you but these days it's not worth quiting a job for it. Speacialy since you have a job in this area where you have contact with with the right people all the time.
Just take a couple of hours once in a while to study for the tests. You still have plenty of time. And once you get close to the test date try and study a little bit more if you can. You don't need to do all the tests every time. Do 2 to 3 different tests a day or something.
For the mental arithmetic there is a couple of tips on some web sites. Take a day off and google them.
There are also a couple of good threads about the tests over here on PPruNe and also with almost all the questions on the General Knowledge test. I don't know where it is anymore but shouldn't be that hard finding it.
If when the date is closer and you realy don't feel ready, contact them to see if you can postpone it or even cancel it. I don't know how much time in advance you need to do this but find out.
Good luck with your choice and if you decide to do it, very good luck with the tests.
