Clarke vs. Sealey vs. Makita

Clarke and sealey are generic cheap tools, real Makita are great, but you may also find makita branded rubbish. For the amount it gets used (IME) I would go for the cheapest you can find.

Do you have a particular use in mind?

Chopping out rusted body sections.

If you are thinking of a 3 inch disc cutter like this: Ebay 12.99 with discs

Then that is not ideal at all, what happens is that the body of the tool gets in the way too much and the discs wear down quite quickly, which means that you run out of cutting ability very fast. BUT at 12.99 it is worth having in your tool kit, also bear in mind that air sanders/grinders use a lot of air and are very noisy.

A standard electric angle grinder with a 4 1/2 or 5 inch cutting disc would be much better and there a makita is definitely the make to get. A multitool with metal cutting blades is very useful too, as is a jigsaw/stabsaw. An air chisel with the right cutting bits is very useful for longer bits, like sills, as is an air scissors or an air nibbler. For odd bits involving many layers with gaps a contra rotating double disc cutter is the tool to have

A plasma cutter is quick and nice to have, as is oxy-acetylene, but a standard electric angle grinder will do 99 percent of cutting out rusty bits.

The above is based on doing the job and having all the above available (including the three inch air cut off tool which scarcely ever gets used for anything)

The other tool that gets a lot of use in these jobs is a narrow belt filer for prep work after cutting out the rusty bits.

Although I have never really done this job on cars, I have used all of these tools apart from Oxy and Plasma, and this description matches my experience exactly.

Mine not quite so much. I find multitools *useless* for this purpose. They're great for trimming copper heating pipes in corners but next to useless for any kind of steel. Likewise jigsaws/reciprocating saws. There seems to be a gap in the market here for a disc cutter that falls in between the cut-off tool the OP mentioned and the die grinder which is just a bit too small for this type of job. If there were such a thing as a HD die-grinder/dremmel with discs of say 1.5"-2" and uprated bearings to cope with higher radial loads, I'd be first in the queue for it. At this point I expect someone will chip in with a link to an existing product matching this description that I'm unaware of which will make me look a right mug, but it's a chance I'm willing to take. ;-)

I tend to agree here.

I've also never had much success with nibblers, most of my cutting has been with a 4.5" angle grinder. I find underseal tends to hamper progress with nibblers.

More recently I have a plasma cutter though only have cut thicker (2-5mm) sections of clean steel. It also leaves a lip which will need removing with an angle grinder but is relatively quiet (white noise), apart from the compressor running!

For the multitool you do need good blades for steel, but it works well. I have even used mine for cutting out an opening in stainless steel. (the circular HSS blade for my one is very expensive, but works well for ages) It might also depend on the multitool make/oscillation arc too as to how well it cuts steel.

But like I said an angle grinder with cutting disc is the primary tool.

I've got a rather rare Hitachi electric nibbler. Assuming you can get access, it works a treat. And fairly safe to use - I'm terrified of angle grinders. ;-)