I have two neutered male cats. Neither of them spray. Oz was sexually mature (and spraying) when we adopted him; the Humane Society had him neutered before we took him home, and he hasn't sprayed since.
Neutering is not the only factor in feline weight gain. Too much food and not enough exercise also factor into it, just as for humans. Spike is far from being a fat cat: at thirteen pounds, he is long and lean, and very, very active. Oz is overweight, but not because he was neutered; he's overweight because he overeats, and we're working on that. (He still has a stray mentality: if there's food out, it has to be consumed right away because there might not be food later on.)
Neutering does stop male cats from spraying and it doesn't necessarily make them fat. Unless you have a pedigreed stud cat (I'm not sure if that's the appropriate term, but I'm not a breeder and have no knowledge of breeding cats), there is absolutely no reason not to neuter your kitty. Some cats may continue spraying (male and female) but I'm not sure why this is, or if age at time of alteration plays a part in this.
As for the smell ... There are lots of products on the market to get rid of the smell and the mess. White vinegar and baking soda also do a good job of it, and both are inexpensive and non-toxic. I'm sure other people on this site can give you excellent recommendations of products that worked for them.