I mowed lawns for years and years....then the clients saw that I bought a Mercedes-Benz and the number of jobs very suddenly dropped off
I forget the make but we have several 1980s "Rear Bagger 21 inch" mowers with commercial grade motors that have proven to be bullet proof.
But yeah don't go too overboard on equipment and search for some good used equipment if you can. Depending on how much you charge per lawn it could end up taking you quite a while to pay yourself back (I used to charge $20-25 a lawn being a young kid...what is the going rate for professional companies?)