It's good for sheetmetal that I know for sure. I also know your welding technique would be better faster with a more advanced mig welder. However, I used the HF welder to make a steel base for an attic A/C unit out of 3/16 angle stock and it is indestructible.
Our car project for me is paced by money, so most times I buy tools I need second or third hand and parts for the car I get from the place with the best value.
Post a wanted ad on here and craigslist saying you want a HF welder. Try it out and see if it's for you. If not you can sell it.
If you take your time and you practice you can do well with the flux core welder. I had plenty of members here always comment this tool is garbage, that part is inferior. You need to only do it this way.....etc etc.
I saw your thread about paint with guys telling you $10K for paint. Dude that's crazy! At least for me it is. Just use your head and have a thick skin when some of the boys try to rain on your parade! Boldly go where no man has gone before Captain!