I was out of it for 7 or 8 years, and was likewise mystified when I came back. I now have a Slink Original body (too many of the others seem to be designed for absurdly large breasts, including the other Slink body) and a Lelutka head. But! No rush. Indeed, if you mostly wear clothes that cover much of your body, the system avatar has some advantages. It's simpler, but also you can use the alpha masks that come with clothes and you're actually probably more likely to get clothes working without skin poking through than you are with a mesh body.
Spend time with demos. I eventually bought *two* Lelutka heads because eventually I decided I didn't like the way the corners of the lips folded on the first one; I didn't notice that right away. (There go those L5000!)
There is no need to go with *the* most popular body or head, but you want *a* popular one. My Slink body isn't the Matraiya body that seems to have most of the mindshare, but nearly everything that has custom sizes for mesh bodies has a Slink size. (This may be in part because the Slink folks are more willing to give out Dev kits. I'm building a small business with some mesh clothes. I applied for Slink, Matraiya, and Belliza Dev kits and only heard back from Slink. Lucky for me that is the body I actually wear myself!)
While the look of the body or head is important, what's more important in choosing the brand is the HUD that comes with it. You'll use this more than you expect; it's not just for one-time configuration. You'll need it often when changing clothes or makeup. Make sure you like and can work with the HUD in the demo. For the body or head, you don't have to take it as-is, but can (usually) modify it with a shape sliders. ("Bento" heads are the mesh heads that do this.). Play a lot with the demos. You also aren't restricted to the skins that come with the body or head, as there are lots of great third party ones out there. (I got mine from YS&YS, and have also used Esode.) (I can't find a reference in world or on the web to the store that I got my old skin from 9 years ago... the world moves on, some things are forgotten!)