I picked up a small stud finder a while back at Tractor supply. It's the kind that uses a couple of those real strong rare earth magnets and you slide it over the wall until it sticks to a nail or screw. It is really a screw finder and if the guy who screwed the sheetrock up was off, you might be off too with your nail or screw..
It does a fantastic job aligning and holding sheet metal parts flush with each other before tacking them in . Works great to hold the tarp on the truck I'm working on when I cover it up for the night.
I picked it up from the bargain bin for $2.99 and I need to go back and buy about 4 more.
The 4-5 inch angle grinder suggestion is very true...I'd have ten of them if I could afford it, one with a grinding disc, a couple with flap discs for finish work, a thin cutoff wheel mounted in one, and one with a knotted heavy duty cup wire wheel. For occasional use the Harbor Fright ones sometimes hold up pretty good for the money but a Dewalt or Milwaukee or similar quality tool will have more umph and run a long, long time. Don't mess around with the smaller arbored ones, get one with the larger coarse thread that most of the discs and wheels fit..
Any time I see vice grips of any kind made by Irwin or a quality manufacturer at a yard sale I grab them. Very useful for sheet metal work, specially if you luck onto a set of the c clamp ones.
A large chunk of railroad track or I beam to use as an anvil, a similar sized piece of heavy angle, and a bunch of cheap cast C clamps from Harbor Fright make a decent poor man's bending brake and sheet metal forming tool. A Magnabend would be nice to have but at $2000 I stick with my angle iron.
Gotta have a small try square or the rafter layout one that is a triangle for measuring and laying out patches.. A Sharpie is indispensable for rough layout, and a scratch awl for final scribing of the edges of the patch. Measuring tapes are also mandatory, I keep one in my pickup and one in the shop and one on the dresser and one in the kitchen and one on my sign machine, etc....
Can you tell I have been in sheet metal up to my elbows lately? Too bad it is a 66 Ford F 100 instead of my poor neglected jeeps.

My buddies welder and plasma cutter are nice too... As is his floor jack , high stands and engine crane.. What I do miss is a nice heavy workbench with a good sized vice.