Hi Fede, if you just want to get started duluthboats is right vue spirit is good for the cost, is easy to use and fast in rendering.
I use before Rhinoman and BMRT, free no cost, but slow, but quality is probably better than most of low cost software around, now I use carrara Pro v4 some jobs paid for it, easy to use, reads most 3D CAD formats, and is very fast, very complete software for the cost.
if you are looking to start rendering for business then XSI foundation is your best bet, average cost, strong rendering engine and very fast, this to start taking small rendering jobs, I have the demo and the interface is not so friendly but I model everything on rhino and then export to XSI so is ok, I don't recommend flamingo I think the price is high for what it offers XSI is much more complete and better renderer even carrara is better, I read that lightwave is introducing a version called inspire 3D it supposed to be a low version of lightwave, worth to have a look.
C4D has an old version that was for sale for 150 bucks, if you call them they may sell this version, is good it works great and there is many resources on the net for the software.
And if all you are looking to start on your own as a hobby meaning that you have most of the time to learn and practice then download Blender 3D and that's it, play all you want and render all you want, the interface is not friendly at all but is free, it works and is very well supported.
it all depend what you goal is (besides nice pictures) if you want to offer some services XSI foundation will be my choice to start rendering services, then carrara Pro V4, if is only to learn but you want to start with a good program then Look for the low version of C4D, find out about inspire 3D, all this leaving the high end packages a side (or at least those with exaggerated prices).
Learning curve is not so bad in most of this packages, softwares like carrara have a wizard with all the bells and wisthles to prepare your scene for rendering so even myself can render
good luck