Planning on leasing a dedicated server. Can I resell space and host web sites on it for clients?

Hello, my friend!
The answer to your question is of course you can!
The problem is that you will have to install and setup everything (by that i mean apache server, php and mySQL) yourself!
Also don’t forget that is something goes wrong - it is your responsibility.
In general it is not a very safe nor reliable. Unless you manage to strike some sweet deal with the guys you are leasing from, like auto backups and soft install upgrade…
In any case the short answer is yes.
The shorter is that it is not worth it - since there is just so many out there that do it:
hostgator
bluehost
hostingzoom
and many others!
you can’t beat their prices…