Hi and welcome,
You are totally correct in your post, and you could work with xcart to get it up and running just looking at knowing html and some smarty (for minor mods). However PHP and MYSQL knowledge will really help you understand how and why it does what it does and how you could tailor it to suit your needs.
It will also help with understanding smarty aswell
Limiting the damage that a client can do to the admin side is a good idea and can be easily achecived through removing the links to options where these things are done.
Pricing for customer groups - yep - all possible.
Have fun and use the forum for your help with your first xcart like the rest of us did.