For used bikes it really depends on how the bikes were ridden and how well maintained they were. Both 105 and Force work fine, there are differences in ergonomics and the way they shift, but you'll be a lot happier on a bike in good condition of either setup rather than one that has been ridden into the grave.
Edited to add: You don't need to earn a nice bike other than earning the money to buy it with. The cheaper groupsets tend to have 99% of the functionality of the expensive ones. Pro riders would still be way faster than people here even if they were on the lowest entry level bike and we were on $20000 bikes.
Last edited by milkbaby; 02-13-15 at 04:15 PM.