If your bike position looks like that (much higher handlebars than seat), that will really slow you down. An upright position adds a lot to wind resistance and the drag is what slows you down. In fact the faster you go, the greater resistance to overcome.
So you probably are looking at a road bike if you want speed.
Also ignore those alleged $50 deals on CL. When someone comes across something like that, it's likely because the seller doesn't know how much the bike is worth. But that's about as common as a person finding a 1966 Cobra in a barn and the owner wants to sell for $500. Most often a $50 bike is ten times as bad as a $300 bike.