You can rent time for a Mac in the cloud to build your iOS apps. You can test in the built in xcode emulator. http://www.xcodeclub.com/ $30 a month and you can cancel any time. I'm not affiliated with them in any way, but I've known devs who use them. It can be much better than putting much more upfront at once to buy a computer to do builds on.
You could crowd source testing too... just have to find people with devices willing to help test. Reddit can be a place to look. You can get testers for iOS/Android for sure. The main thing you want people to report is performance + crashing issues.
Defold seems to be really generally solid. The biggest things you need to worry about is not having too costly processing, and not using too much memory.
I've tested Leapdroid and it seems to be a very good Android emulator. There is Nox and Bluestacks too that I've used. Strangely I think Defold builds are crashing in Nox atm, but they are working in Leapdroid / Bluestacks. Leapdroid may be best bet for testing Android for IAP / ads and so on. You can get Android tablets that are pretty good for pretty cheap such as this for $50 https://www.amazon.com/Amazon-Fire-7-Inch-Tablet-8GB/dp/B00TSUGXKE (they are root / cfw able depending on what os version is installed so if you care look that up first before updating it if you get one). There are also cheaper Android devices which are decent for testing just because you are testing relative min specs, but emulators are good to test on if you are only testing features.