Tr1umph would be a great PA product as a foundation for a natural stack. If you have protein covered, I would just stick to the basics since your budget is limited. You can still do a lot with $100 though.
First one on the list would be a quality creatine product. I'd recommend Powermax XT which contains a full serving of creatine as well as additional ergogenic daily staples. Powermax XT can be used as a non-stim preworkout and as a daily staple supplement, and its packed with quality ingredients that can boost recovery and help with performance.
Other than a solid protein and creatine, I would add in a fish oil or joint support product. A product like this can be had for a relatively low cost and most retailers, just make sure its from a quality trusted brand.
The last category of essentials would be a stim preworkout products. There aren't essential, but can make a big difference in strength, performance, and recovery. I would recommend Conqu3r Unleashed, Hypermax XT, and Mesomorph as my favorites. All are great choices.
Also keep in mind - Performax Labs products are currently on sale over at Nutriverse - 20% off the entire product line through August 1