You really need to sit down & do your math carefully.
I have a customer who does nothing but pennsioners lawns on a government contract.
He uses Honda residential walk behinds, Honda line trimmer, Stihl blower, Shindawa chain saw, Atom edger & a local Greenfields 28" domesic ride on.
The Greenfields run a cork plate clutch & drive system and has prooved very reliable.
All of his gear is second hand & I do all the scheduled maintenance on it and to date only break downs are belts.
He makes a very good living, goes to FiJi or Hawaii every year for a 2 week vacation and has a fairly good life.
Another customer decided he was going to do mowing as a second job, walked into a glass front and walked out with $ 20,000 worth of debt which over 3 years is still being payed off.
All commercial grade Husqvarna gear and the only thing worth buying ( IMHO ) was the chainsaw. His repair bills are greater than the others scheduled maintanance.
If the gear you have is doing th job and making you money, then stick with it for now.
And put your expansion efforts into customers you can service with the equipment you currently own.
Put 1/2 to 1/3 of the PROFIT you are making aside and keep it for equipment upgrades.
At the end of the season have a good look at how much you have in your kitty.
This is your equipment budget, bought outright , leased or paid off, this is all you have to spend.
Do not buy an expensive piece of kit in order to win a contract unless you can amortise that cost with your existing customer base as the new client can give you the flick in a heartbeat then you are working your first job to pay for the second.
Because the working conditions in lawn care are highly variable you will most likely find no one mower will do everything as Ric has already advised.
Long term you will have to decide weather you are going to continue doing lawns to use the spare daylight hours you have or if you can make a better living tossing the full time job for your mowing business.
If you decide to go the first way then keep the costs down. Look at what is in the back of other mowing contractors trailers in your area, try some second hand gear, Usually you can get some one season and flog it off the next for near what you paid for it.
Once you have decided what works best for you then consider pulling the trigger on new mowers.
Note I said mowers plural, you will need back ups for everything because customers will not wear you not turing up because your mower is not working and mowing a 1/2 acre yard with a push mower because your ride on is broken is a pain, doing it 12 times a week is a lot of hard work particularly if you have 8 hours to put in latter that night.
Also remember while it is beer money you can be really flexiable with pricing. Once it becomes mortgage money things change, quickly.