Reply 1: You can go with the Express one for your deployment. In fact, the limitation is a soft one, and only impacts on the DB size stored by vCenter (Express R2 now have a 10GB limit DB size) and performance. You CAN have more than 50 VMs / 5 ESX, but if you have a problem with your vCenter service (i.e. vCenter not starting, slow performance when accessing the inventory, etc), VMware will not support you in this case. VMs will not be affected by this.
Reply 2: Yes, SQL Express come bundled on the VC installation. It installs with vCenter (you can choose this during the installation process).