We built SeriousMD to work with what you have.
Many of the doctors using the app now got started by grabbing an iPad that was just lying around the house and made it to a useful business tool.
You can use an old laptop to load SeriousMD on the browser.
You can use your iPhone that you are already carrying with you.
There's really not much hardware cost unless you want to fully equip yourself and your staff with the latest devices.
If there's one thing you might need to buy, it's an Airprint capable printer. That solves your wireless printing needs from your iOS devices.