> But when I start to blog I find the lack of live preview and spellcheck inconvenient.

What platform are you using Simplenote on? On macOS at least, you can write in Markdown and preview it with Cmd+Shift+P. Spellcheck also seems to work for me, but maybe that's a macOS thing and not specifically a Simplenote thing.

Disclosure: I work at Automattic, but not on Simplenote.