Will my old messages appear on a newly synced apple device? If I sync my icloud to another apple device, such as an iPad - Will the messages that are already on my iPhone appear on the iPad or will it only be newly received or outgoing messages that appear? 

Also if it is the case that my messages on my iPhone already will appear, will texts that I have deleted from the iPhone appear on the iPad once synced. 

Hope that makes some sense!
"synced" can mean differnt things and is slightly ambiguous.  

However if you "backup" your phone via iCloud or iTunes, then you can restore the backup to another device and your iMessages and SMS texts will have been transfered.   

More info here: iCloud: iCloud storage and backup overview