I'm also new to off road racing and started with sct in rookie class with 17.5. Lots of fun but more like demolition derby. A month or so after I graduated rookie I picked up buggy 17.5. First night out , I was faster with my sct, but by the second race day I was faster in buggy. Buggy is super fun and precise. Now I race both.
The nice thing about the way I did it is I learned some basic driving skills with the sct without breaking the buggy constantly. So, I'm glad I started sct rookie. Now in buggy, I just try to stay out of the fast guys way and follow them around the track the best I can. It's very competitive.
Good luck to you, I'm addicted to the sport and surprise myself that I read these forums even.
In terms of expense, it's the lexan body cost of short course that's expensive. My tires and foams have lasted a long time at my track, but they do cost more initially. I bought my stuff used and rebuilt both cars with lots if care.