I own a Honda 400ex, which I do ride frequently (its the only thing I own with lights...)
MY YZ125 will outrun the Quad with ease and feel much more comfortable doing it. I expect that you will find that any dirt bike will provide greater speed than you are used to from your quad. Where a quad will shine is in soft sand or crawling over insane boulders.
If you are reluctant to do engine work then I would advise against buying a "MX" bike. Perhaps a bit of definition is in order here. The major bike brands will make two or more different product lines of dirt bikes.
The MX bikes are made for racing and are no frills (such as kickstands or headlights) and as light and fast as possible. The engines are "high performance", usually requiring high octane gas (91 octane) and when you push them to their limits they don't last long. The owner's manual will tell you that the piston needs to be changed on a regular basis.
The "off road" bikes are made for more general trail riding. Their engines are generally of lower compression, allowing them to use regular gas. They are detuned slightly so that they run cleaner (environmental issues) and will also have lower internal temperatures which makes the engines survive a lot better. The bikes will often have lower seat height and less suspension travel so they will not be as great for jumps. Electric start is fairly standard on an "off road" model.
I have a hard time picturing a 21 year old who describes himself as "suicidaly aggressive" on an "off road" bike so I would recommend either a MX bike or one of the cross overs, such as the Yamaha WR450F. You had better learn to do engine work, however, as it won't last long if you don't maintain it.
As others have implied, you should also consider upgrading your protective equipment. I assume that you already have a helmet. Unlike riding a quad, when you ride a dirt bike aggressively the odds are very high that you will go down. A good set of boots, gloves, chest protector and kneed pads will go a long way to helping you get back up again.
Rod