I like your approach to the industry. I have a similar setup, started small and paid cash for everything, and then worked my way up and grew the company. I started when I was 14, with a 21 in push mower. I now have a 36" Metro that I just bought this past fall--best decision I've ever made. It isn't as productive as the larger mowers but it works well for my operation. It takes a few more mins to cut a lawn but I'm not spending money that I don't have, its all profit.
If you can afford it, I would highly reccommend starting off with a 36" walk behind. I always buy slightly used, typically off craigslist. Research and timing pays off. Just today I saw a 36" for a great price, good time to buy.