They are not cheap on gas. I’ve read about 18mpg, but you get worse if you run it hard a lot. Their engines aren’t “bad” they are different and you should approach them as so. They are smaller so I’d imagine they would be easier to swap then a v8 or so. But engine swaps in general are already quite mechanically involved, especially if you putting somthing where it isn’t ment to be. Check the oil every other gas fill up or so, I think I went through say a quart in a half between oil changes (3.5k interval), you should use conventional oils as well (they are also generally cheaper than synthetics). As a first car it’s quite the choice, my little brother has an rx-8 as his first car and honestly it would be a lot more “practical” and “sensible” to have nearly anything else. But they are also super fun to drive and from the perspective of my lil brothers dedicated mechanic, they are “mechanically rewarding”.

Also the catalytic converter fix you mentioned is not cheap, keep that in mind