It all depends on what you’re trying to render, but I would consider bigger RAM than the 16gb.
Thea is not a speedy render as it is mainly unbiased. It has a biased engine (BSD), that behaves similarly to Vray’s, but where it stands out is on the Unbiased engines that most users tend to favour (I never used BSD for production though I tried it, I don’t like it).
Unbiased engines are not fast, they are simple to use and deliver ultimate quality though. Thea is one of the fastest.
Every Thea engine works on the CPU so it’s understandable that a nice CPU is the recommended setting and a decent amount of RAM will allow you to render most projects. 16GB, is imho, small. If you intend on rendering exteriors with massive vegetation and 4k textures or multiple proxies and at high resolutions, you might be bottlenecked. There are a couple of very nice Thea features like Relight, or export channel passes to post edit in image editors or compositors, that will consume even more RAM.
Where Thea shines though is at Presto which supports both CPU and GPU.
Presto is also an unbiased engine, but it will get it’s speed mostly from the CPU+GPU collaboration. The GPU, as I said before and as you can look at the benchmarks, usually contributes with more samples per pass than the CPU. The Top GTX GPU is much cheaper than the Top CPU too. Also you can get more GPU’s inside a Desktop than CPU’s so it’s a better investment to think on GPU rendering than CPU, in my opinion.
However the GPU is also bottlenecked as you can only render jobs with the max amount of VRAM you have. If a GPU is out of VRAM it get’s out of the loop and on tougher jobs only the CPU will hold as it will be able to have 16Gb, 32, 64 or 128 and even more. No GPU can follow there.
That’s why, since I bought mine with 12Gb, I will never go much lower than that. A 1080Ti with 11Gb would be the only one I’d consider from the ones available, besides the costier Titan X.