The best lens to get is one that will minimize distortion. Professional lenses (minimum f2.8 aperture) use higher tolerances in glass. You get what you pay for. If you get a zoom lens, cheaper ones have more distortion at the extremes: barrel distortion at the wide end and pincushion at the tele end.
Aside from the lens itself, the distance you stand from the model also results in perspective distortion. The closer you get, the more exaggerated the size of close object will appear, in comparison to more distant elements. If you're doing a head and shoulders, with hands folded in front, the hands will appear too large. in comparison to the head, if you stand too close.
You should be about 10 feet (a little further is better) for head and shoulders) to avoid this. At least 20 feet for full figure or more. You have to look and see what your image looks like.
The wider the lens the smaller your subject will be in the frame compared to a longer lens, so you need a lens that will allow you to frame your subject without going too far beyond the area you need. Otherwise you'll have no detail if you have to blow the image up too much. Just remember that you'll need adequate amount of space to position yourself in relation to your subject.
The best thing to do is go to a photo store, stand the proper distance away from the salesperson and see which lens works best. Different cameras have different size sensors, so checking it out personally is the best way.
It will be difficult to buy one lens that will accommodate all your needs unless you purchase a zoom lens. The problem with zoom lenses is that the less expensive ones are very suspect, image quality wise.
The thing to keep in mind is that certain degrees of distortion may be acceptable for photographs but when converted to paintings they may look weird. Camera experts may not take this into consideration so take what salesmen or photographers say with a grain of salt. What's good for them may not be good for you.
Budget is always an issue but spending a little more now may allow you to avoid repurchasing later.
If you buy a nikon camera (my preference) this website, in my opinion, gives the most objective analysis for nikon lenses:
http://www.naturfotograf.com/index2.html
Good luck