I chose to skim over most of the responses as everyone has a different opinion. $13,500 is more than enough to start a business assuming you have a truck to with. Buy a new commercial grade weedwacker, backpack blower, and hedge trimmer. Buy a used or new trailer 5x10 to a 6.5x12, it all depends on what size mowers you need.
Your biggest decision is what size mowers to buy, do the properties in your zip code have small gates that only a 22" can fit in? can only a 36" fit? and so on.
Most people already have a 22" mower so just use that as long as the cut is acceptable. If you can fit a 48" mower through most gates in your zip code then get a 48" mower, if not you NEED a 36" mower or you will waste time with the 22". If you have wide open property in most of your zip code you can get a 52" or 60" mower.
Look on craigslist and eBay for deals on used equipment in your area, get commercial equipment with under 1200 hrs because it should last you at least 3 seasons before hydros need to be replaced or other expensive components.
22" to 36" mower should be a walk-behind, i prefer belt driven 36" over hydro
48" to 52" can be walk-behinds, but I prefer stand-on mowers. Once again if you are in wide-open areas then ztr's would be fine as well.
60" and up is when you should be a ztr
Everything is different for your application.
THERE HAVE BEEN 4 THREADS IN THE PAST WEEK ON EXACT MOWER SIZE AND COST TO BUY. Look them up in the search bar.
Look professional and plan on spending $500-$2000 in advertising, I advise postcards.
(For a trailer, echo/stihl weedwacker, blower, hedge trimmer, 22" mower, and 36" mower you will be in the $3500-$6000 brand new. Look into World Lawn Mowers for a cheap new walk behind, I have heard good reviews and my local shop just started selling them this year.)
There is an Old Blue Chair for each and every one of us, we just have to find it.
2006 GMC Sierra 2500hd crew cab
BOSS 8'2" Power V
Hustler X-one 60"
Toro Proline 36" wb w/t-bar controls
JS46 22" walk-behind (wouldn't recommend)
Shindaiwa T254, Echo trimmers, chainsaws, pole hedge trimmer, pole saw, hedge trimmer
Shindaiwa 633 and Husqvarna 130BT blower