What class were they usually, poor ? Middle ? Not quite rich ?
How much kit would they have had to provide for themselves ?
How high could a plebian climb in the ranks ?
How likely were they to survive to retirement ?
The thing is that the answers to these questions depend on when precisely in the Roman era you're thinking about. The Roman Military section of the FAQ has answers for most of your questions.
For the rank one in particular, this thread has input from u/angry-mustache and u/PapiriusCursor, though the answers are understandably focused on one particular time period.