Advice so far is good, a lot of people buy the minimum size machine they think will do the job and soon find out they wish they had bought something with a bit more upper power....so have to go through the buying process again.
A gas/gasless machine is a good buy because it gives you the option of both, welding outside can sometimes mean having to use gasless to avoid the gas being blown away, although a nice day and/or a screen often allows you to stay with gas, which is easier and neater to use on car body material.
I suggest a Clarke 130 to 150 amps would be a better buy for you, you will sooner or later want to make something using material above car body thickness.
30 amps minimum is about the norm for most machines and is what most people are working at.
Start practicing on some cleaned up scrap 2mm or 3mm until you get the 'feel' and decent runs.
Invest in an auto darkening mask, it's one of the best aids to learning and even a cheap one off Ebay will do for hobby work.
For gas, have a drive round your local calor gas agents, many now sell welding gas with low rentals, the small disposable bottles work out very costly and don't last long at all.