A kart suit isn't necessarily required to race karts. Some places it may be but a lot of times you can race with jeans and a racing jacket.
Like I wrote above, they are recommended and preferred but not always required.
They come in materials that are abrasive resistant as well as fire resistant. You will want to get one that offers both of these, even though it's not required and karts rarely catch on fire. If you race a caged kart, you will be required to wear a fire repellant one because you are strapped in with a safety harness.
Generally what you wear in your normal clothes will work out well with the same size. To be extra sure, always try on something before you buy it.
You will want it to fit comfortably, but not too baggy. Also make sure it isn't too tight so you are very uncomfortable.
Never use machines to wash or dry your suit. If you need it cleaned, the best thing to do is get it dry cleaned. If you need to wash it, wash it by hand and let it drip dry.
These are a very good investment, since safety is always most important, and plus the suits last a while if you take care of them.