Might be a longshot but check local hardware stores. I found brushes for an old belt sander at mine. They had one of those compartment drawers full of common sizes.
I used some composite decking trim board (waterproof). Sealed it against the floor with urethane caulking and screwed to the studs. My walls are drywall so I put a J channel along the bottom of the drywall and it sits on top of the deck trim which is also 1/2".
You can use type M copper. It's cheaper and will handle the pressure from any typical garage compressor.
Examples -
3/4" soft 346 psi @ 150 degrees.
3/4" rigid 701 psi.
1/2" soft 420 psi.
1/2" rigid 850 psi.
reference https://copper.org/
I used some Gator belts (from Menards) that held up surprisingly well. I even sanded some rough spots on my concrete driveway with 36 grit and never broke a belt.
You can buy metal discs just for this purpose. Various thickness and diameters. I have some but don't remember where I got them. I know it was online because I could not find them locally.
edit - might have been Speedway Motors.
I recommend a Purdy "Dale" style brush. They are angled and tapered and available in a few different sizes. I use 1 1/2" and 2" with really good results. If you have a steady hand, you'll find that with a little practice you can do a lot of trim and cutting without masking or any edge tools.
Sounds like they used a handheld saw instead of a walk-behind saw. Handheld saws are much cheaper and they do the job but they are only as good as the operator's ability to follow a straight line. Getting straight cuts with a walk-behind saw is easy. Sikaflex will work as a filler. I've used it...
I wouldn't be to concerned about getting the entire tank coated. If you can just get the bottom few inches protected, the tank will probably outlast the pump.
I have used two different coatings in motorcycle tanks. Kreem and Red Kote. Good results with both. They hold up to gas and alcohol so I don't think air and moisture would be a problem. They require cleaning and etching prep.
If you buy one, be sure it uses a filter that you can buy locally. The unit I got about 15 years ago had an oddball size filter that had to be purchased from the unit manufacturer. I built a box extension on the inlet so I could use common furnace filters. Works well.
The best results I've had so far is 40 grit heavy duty sandpaper on my small 3x21 beltsander. It actually leaves a nicer finish but I'm not going to do the whole 20' x 60' drive with something that small. I'm thinking about renting one of those big floor sanders. What could possibly go wrong...
I had a new concrete driveway done about 9 months ago. They used a red chalk line to mark the cut lines. According to Irwin, it's "permanent" and it does not wash off. The contractor said that weather will eventually take care of it but now I have a pinkish hue all over my driveway. I'm not fond...