i am well aware that when using SP-Solution Upgrade the sharepoint server throws a error 503. this isnt the problem
what i need to find out without testing is does the 503 affect the entire farm or is it web application specific?
Its webapp specific but sometimes can effect the farm during deployment so users can breifly see the 503 error! If its not gone by the time its done you need to check your Application pool on iis to see whats not running!
You shouldnt be doing any updating unless your users would know about upgrading on certin days and time, usually at nite when no users are on or minimal users are on!