I (personally) would say that Stacks is as close to a need when it comes to being able to build sites how you want them. Similarly, if you want some more freedom in what you can do with your sites, then one of the “frameworks” is a good investment.
The main ones right now are Foundry, Foundation, and UIKit (there are a couple of others as well, but those three seem to be the most prominent). I am partial to Foundation (mostly because it’s the most mature and integrates really well with Big White Duck stacks, which I use a lot) but the others are well-loved by others.
RapidCart Pro is a great way to get a nice looking store setup quickly, though be aware that your client can not edit it themselves without Rapidweaver. The new Cartloom 4 might be a good option for a client who needs to update their store on their own (without Rapidweaver) but it comes with ongoing costs. I’d say most editable online stores will come with some sort of ongoing cost though, it seems…
For clients wanting to blog or update a site themselves (without Rapidweaver) I’d say the three most popular would be Total CMS, Pulse CMS, and Armadillo.
And you absolutely should feel free to build client sites with Rapidweaver. I would say that E-commerce can be a bit trickier for being able to build something and hand it off completely to a client but otherwise, go nuts.
There are some good sales on right now as well for a lot of Rapidweaver stuff, so it’s a good time to pull the trigger.