Should I keep my NA?

Simple. Is your NA a good core to poor money into? Good cage, solid body? If so don't sell the car. But if the base platform is not to your standards get a newer build.

Over the years the build quality just gets better and better on the new builds. There are many fasets to a build. Safety, ergonomics, reliability, easy of service, Aesthetics, performance, add on goodies (cool suit, data, fire system). You have to decide what level you want / can afford.

Very similar to having a 200000 mile 20 year old daily driver vs buying a new car from the dealer. Both will get you back and forth to work. We all want the new shiny one but do we need it? Or do we just want something nicer.

It all depends upon what you want to do. I went down this road last year when I popped the motor on my 1.6. I had to decide what racing I wanted to do. Did I:

Want to continue racing my 1.6 in WDCR-SSM?
Build my 1.6 for regional or majors competition?
Build my 1.6 as a ITA car or as a 1.8 ITA car?
Build my 1.6 as a STL car?

My car is not a top shelf build, but it is a solid, reliable car capable of running up front in SSM. I've owned the car for nearly 20 years and it has always been well maintained and is free from those naggling little issues and hacks from multiple owners and poor care over the years. That was a big factor in the decision making process. I've been fortunate with this car and I've seen too many people having to chase the little issues that come up from years of neglect.

The results.

SM in a '99: I went as far as renting a '99 and running it in a regional. It was the first time I've been on Hoosiers (we still use the RA-1 in SSM) car was a pleasure to drive. Almost a Cadillac compared to the 1.6 to drive, and The Purple Crack is addictive. The finances are daunting. My car is worth about what yours is, so the delta to a new car is the same. Add in the $800 for tires every weekend or so (the Toyos stay fast forever) and it adds up fast.

STL 1.6: I love the light weight the car runs as a 1.6 in the class. I spoke with several engine builders, including Kessler, and when I heard phrases like "the first iteration of the cams will be $1000, and there may be several" I got nervous. Plus, I'm not into the whole engineering aspect of the class.

ITA: The participation numbers in the NE and mid-Atlantic just aren't there. I currently race against 40 cars. Why would I trade that to race against 7?

SM in a 1.6: At the time the 1.6 hadn't been given the allowances it now has. Frankly, I believe that the 1.6 is a 30 year old car (from design, not production start) and that time marches on. The NB is the "easy button" car.

In the end I chose to remain in SSM. You don't have that option in your area. However, given the allowances we've gotten in SM I'm looking very seriously at trying my hand in SM next year.