“I had in an accident” is incorrect. “I had an accident” correct “I was in an accident “ correct and similar. Other than that, it is perfect.

I was involved in an accident today and my car was severely damaged.

In English we often end sentences in a preposition (up), but it is really incorrect English to fo so. Just leave off 'up' and you are grammatically correct, but say it as written and everyone will still understand you.

Yes, your sentence is okay. Charlescuy's version is more formal, but yours is still correct English.