You do not need a class 1 medical to start ATPL ground school, but it is generally recommended to get it over with before you waist to much money on training.
There are 14 ATPL subjects. CATS have divided this into three modules (other schools only spilt into two modules). Before you take the 3-5 exams in a module, you go to the brush up week related to that module. So you go to CATS three times. Other schools have 2 x 2 weeks.
You decide your self how soon after the brush up week you want to take the exams. Theoretically you could wait and do all exams after the last visit to CATS, but I don’t think anybody would recommend that. The brush up weeks are timed in the third week of the month, so you can plan to do the related exams in the first week of the following month. The modules rotate, so that each module brush up week is repeated every third month. You can do them in any order you like. Not necessarily 1 – 2 – 3. And you have 18 month to get it done, so you make you own schedule. It’s all up to you.
You find all the information about the brush-up and exam schedules etc. on the CATS web site, if you log on with their demo user.