(Keeping in mind I am still preparing I think it might help me with a date in mind to work towards.)
Actually, it depends on how motivated you are
Also once I fix a date in advance can I change it later?
Once you've bought a voucher, it has an expiration date which cannot be changed.
Then you can decide when to have the exam (anytime before the voucher expires)
The date you've decided to have the exam can be changed, within certain conditions. At Prometric Centres, I think that you can change the date within 3 business days before the exam day.