A case of penetrating oil. Kroil is a top brand, but WD-40 is not made for this purpose.
The secret about (true) penetrant oils isn't much of a secret: LET THEM PENETRATE! I would douse the car with the stuff, emtying the can each time, if you must, once a week, and then NOT TOUCH it for a month. If you follow that discipline, rounding off, or breaking a fastener will be very rare.
If you're impatient, you either pay the price, or get quite good at drilling out the remains of bolts, studs and screws and retapping them.
Yes, I too agree that heat, when properly applied, will remove just about any stuck or rusted fastener. It can also be more risky, depending on your skill-set and your immediate environment.
Here's a little item I recently picked up for when I need to undo a stubborn fastener in the presence of a car's owner (I hate to stress my customers out by attacking their BMWs with an open flame__though I do laugh at the expressions on their faces while doing so...). It also works MUCH BETTER than a torch, when modifying tools for task-specific purposes__you can pinpoint exactly where you want the heat, to make your bends and offsets.
Likewise, Reid also makes a valid suggestion...
But to answer your original question, if budget approximately $138,000.00 for tools, you'll probably have <span style="font-style: italic">almost</span> everything you need__NO ONE has it all!