If the 28 acres has a lot of open areas, I go with the 60" Lazer Z S series but I'm always going to lean toward the 60's over 52's. I actually prefer Scag and in your situation I think a Tiger Cat would do a great job but you didn't list Scag as an option. The finance offers are awesome right now. I think Exmark is 0% for 36 months and Scag is 0% for 48 months. Perfect for a young guy moving up in business.
Also, if you have a chance to land a 28 acre account you've got to make that work. This is the type of account that can launch a start up business into a real legitimate business. An earlier poster said, "life my friend is all about risk". Another said, "take the risk because you will only fall behind if you don't". They are both ABSOLUTELY RIGHT. When I first started my business, I bought equipment for one job... on many occasions. I took on jobs that were actually way too big for me. I hired guys and then I worked my butt off to make sure that the client was happy. I take risks and I've been rewarded for those risks. To me, this is a no-brainer. You'll pay for the new mower in no time with that 28 acre account. Carpe Diem.