Yes that's exactly right, you should test your apps on all devices and operating systems that you want to target, it's not a rule, but is advisable, I'm sure that that there are people who only test on the simulators, but this is not a good idea, and Apple advises developers to test on the actual device as well, as thats what they do when reviewing apps submitted to the app store.
Dont rush to buy the iOS developer program, as this cost $99 per year, do all of your early development and testing with the simulator, and only pay for the iOS developer program when you are nearing the completion of your app, and need to do final testing on a device, to make sure its working correctly.
Also do read up on the Apple app review guidlines, as this will tell you all the things that your app needs to comply with, if your app does not comply with these guidlines, then it will be rejected, and will not be allowed in the App store, I have put a link to these guidlines, but unfortunatly you have to be a registered Apple developer to read them.
https://developer.apple.com/appstore/guidelines.html
But if you read Apple's guides in the developer documentationon, there is a lot of tips about what is required from your app to be considered worthy.
https://developer.apple.com/devcenter/ios/index.action
I will give you a very brief list of tips for your app development, it must firstly be more than just a web page browser, it must not try to duplicate any of Apple's own apps, it must not be greedy with the device's memory, or system resources, that also includes not having memory leaks, it must be able to handle low memory warnings from the device, and also handle interuptions from phone calls and messages, but most important, it must not be crap, Apple are getting more and more fussy about the quality of the apps that they want to allow in the app store, this is because they have to review millions of apps every year, and they want to try and keep the quality as high as possible.
Lastly when buying your Mac to develop on, yes it can be a used machine if your on a tight budget, but make sure its not to old, you want to be able to use the latest version of Xcode if you want to develop for iOS6, that means a Mac that is running Lion, or Mountain Lion, also Xcode needs plenty of Ram to work with, so at least 2 gig at minimum, and the bigger the screen the better, as using Xcode can be screen intensive.
Hope this helps some more.
Regards Mark