I think alot of this is all relative, there are some reasonably quiet mowers out there that aren't too bad and some that don't burn too much fuel. I haven't found many of the newer models to be bad about either noise or fuel consumption, well none that I can think of.
Choosing a mower is more about the features you wan't or need for the most part. I use a combination of three Deere mowers, smallest being a JS45 self propelled trimmer mower with front caster wheels, an L130 48" rider and an X749 4wd, 4ws with 3pt hitch, 540 pto and diesel engine. For tough cutting jobs I have a 4520 cab tractor (60hp diesel) with an MX6 brushmower, great for overgrown lots and small pastures. For rocky weed areas I have a DR trimmer mower, I only use it if the weeds or brush is too large for a standard string trimmer, these trimmers are bulky and harder to use. I don't have a ZTR mower because I don't work in places where they are needed, if you have large mowing jobs and that is of prime importance then check these out.
It should be obvious by now that there are lots of different mowers and related equipment available some of it is entry level and some of it is serious commercial equipment with a high price tag. The easiest thing to do in a new lawn business is buy something. The hardest thing to do is to develop a good clientel who pays on que enough to make a good profit. To turn the jobs you need the knowhow and people who will consistently follow through maintaining the customers lawns.
My interest is in design / building of landscapes taking a piece of raw property and making it look it's best. While this takes more equipment and implements it also takes more experience, so the more you do the better you can get. I really enjoy this type of work the most and get lots of satisfaction from transforming a rough lawn into one that has lots of curb appeal.
In conclusion, you can get lost in all the mower choices for sure but I would think spending the time to figure the in's and out's of the business is what you need to focus on. Involvement in the business for some time, building up some contracts and fullfilling these contracts will quickly determine the future equipment purchases you will make.