Are you going to be mulching, bagging, or just throwing the grass? If mulching or bagging you want the biggest engine you can afford. However, my 19hp on my former ztr did a great job until I put on the mulcher. It still worked good but would bog down in tall grass or thick leaves or straw. Get the biggest engine you can afford but that 2000 is a nice chunk of change if you dont need it.
My Lesco 52" hydro runs like a top after lots of hours. So does my Lesco ZTR 60". Both are Kawi motors, heavy gauge steel everything. Keep them clean, grease and oil, don't abuse them to much, and they run a good long time. My Scag Tiger ZTR's frame had to be re-welded so many times it was scary. Oh, and did I mention my Lesco's cost about $2000 less each than the nearest competitor?
The smaller engine would be fine, we don't bag down here we just let it lay. Too much growth to even think about bagging. Go with the cheaper model and keep it in good shape so if you decide later when your established that you need more power you can sell it for more money. Use the extra you saved to buy some advertising or for start up gas money. Trust me it will come in handy.
Have you worked in this industry before? If not, I would recommend you buy a used w/b hydro and save some $$. Then next season you will know if you want to dump the cash in. No need to load up on the most expensive equipment when you first start out-I say this from experience,I thought I needed everything.
I am going to echo KCarothers statements. Everybody wants to bore with a big auger, the biggest baddest machines they can buy. With the proper business plan and model, starting big can be an advantage. For someone that is just starting out, being bigger isnt neccesary better. Start with the basic equipment and expand as the need arises. You will find that the less equipment you have the lower overhead. Big overhead can break a business that is just starting out. You buy it, you have to pay for it, just starting out you dont always have the steady income that will allow you to pay for all those expensive machines that are just setting around and seldom being used. If you have experience in lawncare you might already know what equipment you can get by with until you can afford that badass mower. If you dont have previous experience I suggest that you start slow and work your way up. Dont sink yourself at the start by buying equipment you cant afford and probably dont really need.
spend the extra money for more power. I have the 19hp on my Toro with only a 44'' deck, and I still bog down quite a bit in tall grass. Even though I do mulch the next engine I will get will be at least 23hp.