Mesh bodys, and there are many out there, are not "needed."
I have several, such as SMB, Slink, Toddleedoo, and Tweeneedoo.
Mesh bodies, depending on the body come with skins that you can select directly from a hud that comes with it.
You can also buy skins to apply to the bodies as well.
In order to wear a skintight clothing item with a mesh body, the item must come in an applier form. "Slink applier for Slink. Toddleedoo applier for Toddleedoo. etc.." On the other hand their is an Omega hud 99L, which many creators use that you can buy directly for your mesh body in the case that you use one, that allows you to use practically any Omega applier on that body. Slink, Maitreya, and Toddleedoo versions can be found.
In order for you to wear clothing and not own a Mesh body, it is best to make sure they come with general Linden Lab avatar parts. (These are the ones that have the different icons next to it.)
You can not use an applier for a Mesh body on the original avatar.
There is a lot of clothes out there and many shops that carry clothes to fit almost every body such as Crazy Kitty, and ADN.
Admittingly most shoes are made to fit Slink or TMP(The Mesh Project) feet. In many cases you can find a skin store that has appliers to match your feet to a Linden avatar. One place for example is The Sugar Garden.
Overall my best tip, read info carefully on the description and or image of the product. Also when you go to buy things, some things you click to buy and it will show you all the items that come in the folder/box. You can look at this for an official idea of what appliers and parts you will get with it.
Also, don't be afraid to talk to the creators, and read around the stores, most are openly glad to help you!