G'day Jim and welcome to the forum,
I started 2 years ago with a 2nd hand k93 kit and have never looked back. Luckily I had some money left over from selling a house that kept us going while the business built up though.
Where are you? I think a lot will depend on your area. I cannot compare the k93 and 4 as I haven't operated a 4 but I'm sure those that own one would say it's good. I've become very used to the 3 with a kohler but would be happy to try diesel next.
I reckon you should have good signage and a bit of advertising. Make sure you are SEEN around your area. I often get calls from people who are following me or trying to hail me down in between jobs. A few signs left at punters properties while your working there is also good.
Then you have to offer great service. How you do that is up to you. You will learn that great service doesn't necessarily mean doing every job that comes along. Up here in the mountains, some places are just too dangerous. Some jobs are better suited to a bobcat or excavator. We also suffer a bit when we get extended periods of rain. It would be good to have something to generate some cash during those periods. A website, properly set up can be great and fairly inexpensive as well.
I think you definately need a trencher and posthole digger as most punters seem to associate them with Dingos. If you can get in with concretors, plumbers, landscapers and the odd sparky (what's that? they're all odd?) you'll get work too. I have a sign in the local landscape yard so as the punters order their top soil, dust, roadbase etc, they see my smiling face as they drive home thinking about how they're going to move it.
God luck anyway, you'll get plenty of good advice here. 
Macca - the dirt attacker!
www.malenydingohire.comMaking Hard Work......Easy!