Agree that you should take any mods one step at a time and only after you've had some experience first.
For example, I figured out early that I hated the overly deep water bowl in the 18. So I bought a couple of alternate shallow bowls. But then figured out that I really didn't like/need ANY water bowl. So I got a Firedial metal diffuser plate which is what I now use exclusively. So I'm stuck with 3 unused water bowls.... In contrast, I thought initially about adding the lid hinge, new door and gaskets. But after using, I feel no need for those.
But since you are choosing between a WSM and pellet grill, you probably should think about an ATC. Since it could alter your choice.
While the WSM runs quite stable/even analog, an ATC adds that auto-pilot functionality that is such a big plus for pellet grills. After running my WSM for a while analog, I started to wonder if I just should have gotten a pellet grill instead. Then I got the Smoke X/Billows system and absolutely love it. With the ATC added, I now strongly prefer the WSM over a pellet grill.