I have personally used a 9amp dewalt 120v recip saw with a vector 750watt inverter to cut the bottom rear corner off an xj, (after both batteries crapped out on the dewalt 18v cordless), if you dont bind the blade or force the saw, it has no problem cutting. the inverter is also cheaper than a cordless saw.
btw, I used to love dewalt cordless, BUT the battery attachment tabs DO NOT hold up to use with a recip saw! after a while the batteries will not stay in due to the vibration. And the blade quick release is weak, and after the blade pulls out once, the retaining ball will not retain the blade as well ever again.
this is true of both the cordless and corded dewalt saws.
the dewalt saws are not built heavy enough for metal cutting duty (and sheetmetal is really taxing on saws anyways)
at work, we use these saws every day and have tried several brands.
milwaukee: best corded saw, period. the speed control is separate from the trigger, ,it is pretty smooth, and it is not too heavy (heavy saws are hard on blades, light saws will kick back some if the blade binds or bottoms out instead of bending the tang over), but dont use the orbital option on metal. the quick attach does not let blades pull out, but the quick attach is kind of weak
porter cable corded: heavy, not as durable, speed control by trigger only. harsh vibration, hard on the operator. I will always find the milwaukee instead of using the PC. in short, not a great saw
Dewalt corded, this was my favorite saw when new. It is the lightest saw, I liked the ease of blade release (flip the lever, pull the triggert and the blade pops out onto the floor) and it was smooth. it wore out fast, vibration became terrible, the blade retention sucks (blades pull out of the holder easily, this ruins the blade due to the groove it makes in the tang. I rebuilt this saw once, at the first sign of problems, but it just hasn't been durable. its a good saw for cutting wood and sometimes metal, but I'll never purchase one again.
Dewalt cordless, all the same likes-dislikes as the corded counterpart, but the battery retention is horrible, The dewalt pack may be fine for drills and circular saws, but it is absolutely not up to the task for a recip saw, the vibration is too much for those tiny tabs to deal with. and once they start falling out of the saw any time you pull the trigger, it wont be long before they won't stay in your drill either. I eventually had to tape the pack onto the drill and saw everytime I used them.
recently I bought an 18 volt Bosch drill, circular saw, recip saw kit, I have been very impressed with the tools so far, the battery clips are much better designed than the Dewalt and the saw is much smoother. the other cool thing about the saw is that it has a variable stroke, cant' remember the exact travel, but it is like 3/4 inch or 1.5 inches.
anyhow, I'd say look at an inverter first, don't go less than 750watts continuous, then you can take your cheaper corded power tools with you.
or shop carefully for cordless, pick a good brand, but look at what you are buying and consider how well it might stand up to the type of use you desire.
(and buy an inverter so you can recharge the batteries)
joel