To re-iterate whats been said before. (too late to the conversation ?)
You have done well to wait until you have UKARA number and then not go out and buy the most expensive toys straight away.
The Specna's you chose are good starter guns which you can upgrade or keep as backups in the future. They should perform noticeably better than the rentals as the hops are new and you'll be able to set them properly.
Keep some budget aside for vests to hold the magazines etc, good ankle support boots, LiPo batteries and decent charger. (search for recommended in the forum)
You probably know, your son will need a one piece full face mask.