As a mechanic, you are not going to limit yourself to any particular models of cars unless you own your own shop, or don't want to make any serious money. Ninety percent of the cars out there are average daily drivers, and they are going to naturally be the backbone of your paycheck. It's likely easier to start with a single marque (or a family of them) and learn the trade from there, before you try your hand at a variety of makes and models, unless you're only going to change oil/filter, brakes, and replace tires.
The simple fact is that not everyone drives a sports car, so limiting yourself to a tiny niche market sounds nice, but the reality is that you can't ignore the beaters, family sedans, minivans, trucks, and SUVs...never mind the service manager and dispatcher will tell you what vehicle to work on. Most shops pay you on flat rate hours (except when you're training), so you'll gladly work on the Chevy Impala that pays 3.5 hours for a bunch of work, rather than the Evo owner who might just want an oil and filter change, unless you really know the owner.
I should also add that many a seasoned technician has told me you spend the first ten years paying off your tools and the fancy box that costs as much as a base-model Nissan Versa. The only freebies are specialized stuff that the dealership owns for diagnosis (computers, testing equipment, et cetera), or large items like transmission jacks and engine hoists...you're not taking those home.
I've worked in the automotive service industry since 2003; now I configure and train departments on how to use the software that helps coordinate the dealership's departments, so I travel around the country and help people because I've been there, and more apt with tinkering with computers and software rather than a car. Plus I have patience, which seems to be the most valuable asset in this position.
In the end, do what you like: Talking shop, computers, gadgets, racing, et cetera is always a nice motivator...people know it and appreciate it when you do what you like to do. Like any job, it's going to have up and down days, that's part of life.