Wrote this bit for Grassroots MotorSports Magazine a while back, still true today.
The NC generation Miata is the hot bargain in Miata performance right now. The reasons are simple, they are relatively cheap and plentiful, high performance NC parts supply is greater than ever, with result that on any given day most of our lifts here are filled with NCs getting setup for track, autocross and street fun. Early NC1 (2006-2008) in good shape are in the $5000 range, later NC2 (2009+) in good shape start about $10,000. If the customer is not a racer, not into serious modification, then we suggest a 2009 or newer. Why? The 2009 and newer have a number of upgrades from the factory including a stronger and more reliable motor that includes forged internals and stronger bits throughout. In addition to the stronger motor the NC2 upgrades included stronger transmission, updated front suspension geometry, and even a more aerodynamic nose. So, for those just looking to buy a really nice driver Miata starting at 10k, and then add good suspension and wheels and tires, the NC2 is a super reliable choice.
The serious hotrod Miata guys go with the earlier NC1 of 2006 to 2008. The NC1 is like buying an empty computer case, you buy it cheap knowing you are going to rip out the worn stock bits and put in only the best hotrod fun stuff. The NC1 stock 2.0 liter motors often failed relatively young and we have had numerous customers get an NC1 with failed motor on Craigslist for as little as $1400 and have it delivered to us for 2.5 liter conversions, Ohlins coilovers, complete stainless header and exhaust system, track worthy cooling system, roll bar, etc. Stock NC motor is 2.0 liter but Mazda's own corporate 2.5 liter motors are a relatively easy install into the NC, nearly plug and play. The 2.5 liter was the high volume motor that Mazda put in every other vehicle they made from Mazda3 and Mazda6 through their various SUVs. As a result of that volume, the 2.5 motor is actually LESS cost to buy brand new than a replacement 2.0 liter, making the upgrade an enticing choice (the 2.5 is also readily available used in local yards under the Mazda and Ford labels for just a few hundred bucks). The 2.5 install can be done with all factory smog equipment for those seeking a street legal hotrod Miata with more torque than the factory ever intended, and it can also be built as a track toy tuned on E85 and making over 200 HP in normally aspirated form, far beyond 300 hp with boost. Fortunately, lots of extra power is right at home in the NC chassis which did not get much respect when new because it had the relatively heavy big bones of a shortened RX8 platform. For hotrod purposes, those big bones from the bigger and heavier and more powerful RX8 means the stout NC platform does not twist into a pretzel when you run it with 200 to 300 hp.
NC Cooling Needs. Proper cooling is a priority even for just street fun. As these cars age, the factory plastic overflow tank is a common failure point because the plastic gets brittle and cracks. Unlike the earlier cars where that just meant a messy and unsightly leak, the NC relies on the plastic expansion tank to be a pressurized part of the cooling system, so when that plastic tank cracks the cooling system loses pressure and the engine overheats. We get a regular supply of NC arriving on flat beds with failed motors due to this simple overflow plastic tank failure. We now offer an upgraded aluminum expansion tank to cure the issue. For those building for track use we also suggest our new triple pass radiator and oil cooler, particularly important for 2.5 liter conversion customers.
Shopping for an NC. Pull the oil cap and check for sludge build up that reveals a lack of regular oil changes, keep walking if you see it. Filthy engine bay also usually means lack of maintenance, just move along to looking at the next NC since there are plenty available. Before you get serious on any NC, have a local shop put it on the lift and check engine and trans and diff for oil leaks along with the underside condition generally (lowered cars often show a lot of speed bump damage). Check color of brake fluid. Check to see if A/C blows cold. Check interior and exterior top condition (replacement tops are available for reasonable cost but the install is best left to professionals if you care about the results looking taut).
Wheel fitment. A 235/40/17 fits in stock fenders on 17x8 to 17x9 with offset of 48 to 50mm. Up to 255/40/17 are used regularly on 17x9 by those racing STR class autocross and track days, fitment requires rolling fender lips carefully up to flat inside. We have fit 17x10 in fully rolled fenders (shown below) with 255mm tires, and we can fit up to 285/30/18 and more with our fender flares (second pic below).
Brakes. The cheap upgrade is factory RX8 brakes which bolt into NC but they are heavy! We worked with Wilwood on a front/rear combo of 12.88 brakes that saves 10 pounds of unsprung weight at the front, and matched rear setup that includes dedicated light rear parking brake caliper. The balance and confidence the full setup provides in late brake passing on track days will put a big silly grin on any face.