Do you have any small engine repair shops in your town already? If you do, are those repair shops doing a good job for their customers or are customers not happy with them?
I know in my area there was a great need for a GOOD small engine repair shop. We have had a couple shops close within the last year or two due to financial issues and poor service. We recently had another shop open within the last couple months, but I haven't heard anything good or bad about them yet.
You will definately have to know what you are doing. There is a big difference between working on your own equipment and working on other people's equipment. You are going to be the guy that people come to to fix their problems, so you really need to know what you are doing. You aren't going to get a good reputation by turning away lots of business because you don't know how to fix the problem. Obviously, there are going to be some things that you won't be able to do, but those problems should be few and far between. So you really need to know what you are doing and be sure that you are familiar with all sorts of equipment, from string trimmers to lawn mowers to snow blowers.
Don't get in over your head. People are looking for quality repairs in a reasonable amount of time. Customers don't want to wait 2-3 weeks to get their lawn mower back, they want it back within a week and they want it done right the first time (no one wants to keep going back to the repair shop for the same thing over and over). So make sure you will be taking on a reasonable amount of work and bite off more than you can chew.
Make sure you find a building that can function as a repair facility. You may not have the funds that some of these large dealers do who build million dollar facilities, but something that is functional for you and the customer.
Don't offer to much all at once, take baby steps. Start out as just a repair facility and maybe offer some small items for sale, like oil, fuel stabilizers, trimmer sting and lawn mower blades. Those items can keep people coming in to the shop and keep your name in front of customers regularly (important when you want to grow your business down the road). Selling those small items also are a relatively inexpensive investment, so you don't need to drop tons of money into selling these things. Wait until you are more well established (5 years down the road) to start selling new equipment.
Just make sure to do enough research in your area to make sure that this service can be supported in your area. And remember to take baby steps, don't try to be some big company when you are not. Slow and steady growth will benefit you far more than quick and rapid growth.
'07 Chevy 2500HD
2010 John Deere Z925A 54" MOD
2011 John Deere Quik Trak 647A
John Deere walkbehind