You absolutely want to get the mechanical grip and balance sorted before you start hanging aero on the car, that's for sure. Without that as a basis, any aero you hang on the car is going to create some seriously unpredictable behaviour.
First thing to keep in mind is that the mechanical grip portion of things is present with and without aero loading. If the car pushes on corner entry, you can correct it with aero by adding a ton of front downforce, and tune it beautifully for mid-speed corners. That exact same tuning, though, on a high-speed sweeper will wind up OVERsteering like crazy. That will give you a car that's very tight on slow corners (not enough speed for the aero to really add loading, so it's all mechanical grip), nicely balanced on mid-speed corners, and then loose as hell on the high-speed corners. Not exactly what anybody would really like to drive, eh?
Assuming you get your car sorted on a pure mechanical grip basis, then next thing would be to sit down and seriously think about what you want the aero to really do. Is the car just understeering a bit on the high-speed corners? Small splitter may do the trick, with nothing else. If it's tight in, but loose out (again on the high-speed stuff), then you'll be looking for some wing to balance the rear, and may want a larger splitter blade as well to cement the front end to the ground. This is where tunability comes in.
Ideally, you'll want the aero gear to mimic the mechanical grip characteristics and balance, so that the car behaves the same in slow, medium and high-speed corners, and in all phases of the corner, as well. Your tuning capability is going to depend on the design of your hardware, but suffice it to say that tuning a splitter trackside isn't the easiest thing. Exposed surface area is your gross tuning. A larger blade (as a rule of thumb) will give you more downforce, so if your tuning is biased towards too much front loading, the "tuning" is really limited to carving the splitter down in size. A good wing (G-stream, AJ Hartman, Brooks Motorsport, etc) will let you adjust the angle of attack (AOA) to balance the front/rear aero very easily, and will make tuning the splitter a lot easier than doing so with a fixed wing. Essentially, it's dial-a-grip with a pair of turnbuckles. FWIW, G-Stream does have a splitter setup for the S197, but it isn't cheap...
Before you say "just give me the x-tra large blade, please!" you have to remember that the blade size is going to drive the rate at which grip comes in, as well as the absolute amount of grip it'll generate. It also adds dramatically to the overall drag coefficient. You want JUST enough blade to do the trick, and no more.
Think slow-, medium- and high-speed corners. Slow corners are going to be in an airflow speed range where neither the splitter nor wing are really going to be contributing anything. Pure mechanical grip. Medium-speed corners, say between 60-85mph apex speeds, will have some aero effect, but you're not going to get hundreds of pounds of downforce without serious drag. If the blade is too large, it's already developing a large amount of downforce at these speeds, but the wing may not be in it's zone of efficiency, so you have an aero imbalance. High-speed corners are where the aero really shines. 90+mph apex speeds, and the downforce is adding a LOT of grip. The wing is really working at this point, and hopefully the splitter is working equally well. Again, if it's too large, you'll have a ton of AOA dialled into the wing, which will create a bunch of drag, and you may find yourself going slower overall than you were without aero... Not what you're trying to accomplish, I'm sure!
I would start with either a splitter from a reputable company, like G-Stream, or roll your own out of plywood, and pair it with a good, adjustable wing, up and out of the dirty air rolling off the greenhouse. If you roll your own, start with one that's just wayyy too big, and keep a sabre saw with you on your testing days. Keep carving it down, and keep backing off the wing until the balance mimics the mechanical tendencies of the car. Once you have the blade at "just right," you can play around with spats, fences and cunards to help fine-tune the center-of-pressure so that the car rotates the same in and out of aero effect. At that point, all your adjustments come down to a degree or two of AOA on the rear wing to fine-tune the balance at each track. If you use a pre-built splitter, you can skip everything but the wing tuning and drag reduction stuff.
Personally, I'd vote for the roll-your-own. As you tune it, you'll learn a whole lot more about the car and aero in general than you would just bolting on a part. Plan on spending one or two complete track days just doing tuning, AFTER you have the mechanical balance stuff down. Resist the urge to "play," and focus on copious, clean note taking, along with whatever data you can log on-track. Pay attention to how the setup under test behaves under braking, trail-braking, maintenance throttle (apex) and at corner exit. Splitters are somewhat sensitive to ride-height, and you may wind up wanting to increase your spring rate to compensate for the increased downforce as part of the tuning process. If you run 200lb rear springs, and your wing is generating 200lbs of downforce, your rear suspension will compress a full half-inch more than normal as a result.
You'll get more benefit from the aero the faster you're going. But, the faster you're going, the more drag the aero creates, and the more horsepower you need to puch a hole through the air at any given speed. A basic splitter and wing is fairly simple to add and effective, but the less you run (blade size, wing AOA) the better off you are. Focus on tuning the aero on the high-speed corners to mimic the mechanical grip of the low speed corners, and you'll be in the ballpark.
The bottom line is that aero is not something you want to rush into. Make sure the basics (spring rate, damping curves, tire pressures, alignment angles, bar rates, etc.) are right, and also that you have a problem that needs correction. If the car feels just fine in both a tight right-hander at 45mph, and a big, left-hand sweeper at 105mph, then leave well enough alone, and don't take the drag penalty on your top speed. If, on the other hand, if the car feels great on the tight corner, but just doesn't want to turn-in on that sweeper, or the rear end tries to swap ends on a high-speed exit, then you might find some benefit from the aero bits.
