hiya mate...
with reference to mig power, I would suggest getting a larger unit every time.
A smaller mig will be of limited use and will not weld well....a larger mig will weld nicer and be far more controlled and can be turned down to do lighter jobs...
I have a 185amp murex tradesmig, running 1.6mm or 1.8mm and Argoshield gas, that I weld everything from thin rusty bodywork up to 6mm thick steel...
Being larger its duty cycle is better so will run pretty much forever on lower settings, wheras small migs will only run for a few minutes at a time if you try to do anything heavier with it..
I also have a snap-on 150a mig, much smaller in size....but the bigger mig still welds thin stuff nicer...this mig i have now converted for alluminium welding with alley wire and pure argon.
I think it is the same old theory that it is much more controlled banging a nail in gently with a big hammer, than swinging wildly at it with a small one!!
My recommendation would be to buy a mig between 100a and 150a which will be light enough to move around without a back injury, will be able to weld most things that you need it to and will give a lovely nice controlled weld on your thinner bodywork...
More important is to turn down the current on it when welding thin steel and trying not to get too much heat into the workpiece...changing to a thinner wire can also help...
I would practise off the car first as welding upside down is a bit more tricky,
invest in a decent auto tint welding mask from the outset...I paid £200 for my mask a good few years back and its brill ( a small price to pay for the sake of your eyes), but they are now available for £50ish or less, and it will much easier learning to weld without having to keep flipping a manual mask up and down all the time...
Also if you are going to be welding outside, you might do better using flux coated mig wire than a gas bottle, as outdoors the wind tends to blow the gas away from your weld, wheras the flux coated still gets it in where it is needed...
Last thing you must buy before welding you sills is a FIRE EXTINGUISHER...and its a good idea to have someone else there who can keep an eye out for you to ensure nothing is on fire (you wont see it while you are welding and when you stop it could be too late!!)
Hope this helps you matey
Gregster