an arduino controlled teenager could be the solution.... 
but...I don't want to make an rc landmower, but an autonomous one
I have a picture of my arduino board, with the motor control shield and the motor I found (I need to find one more)
20120312_074838 | arduino and motor | arduinomarce | Flickr.
The guy that gave me this, took it from an electric bike. Would my motor shield make it work?
...in front of your posts..I have raised my budget to...what is needed to have good stuff (even if...recycle is what I prefere).
My landmower is quite heavy, like the one in the picture (it's not mine, but very similar to mine)
thank you for all your efforts in helping me