A nut or any kind of weight on a string works. You can hear very clearly when it hits water. Then just pull it out and measure. I’ll bet it’s not more than 50’ down. 30 gpm is way more than you’ll need or want to buy a pump for ($$$$). On a well that productive, I’d consider setting the pump so there’s 50’ or so of water over it and call it good. Makes it easier and cheaper to install and repair and you’ll never run it dry with a 3/4 hp pump. Drillers Service has the 250# poly in 100’ increments. Just use 1” pipe for both the drop pipe and the underground lateral piping. There are tables that are in the pump catalogs that will tell you what wire size to use for a given horsepower pump up to a max distance. I wouldn’t be concerned so much about using a Grundfos or Goulds pump. Just get one with a Franklin motor. I pulled one out a couple of years ago, an Aermotor, IIRC that had only quit because the torque arrestor had fallen apart and the drop wire chafed through that was I had put in 35 years previously. Client had raised 5 kids, 4 of them daughters in that time, so I’m sure it had pumped a lot of water. The pump in my own well is a Red Jacket and it’s about 25 years old. Both pumps have Franklin motors, although Aermotor now uses a Pentek motor. I’d probably put in a 3/4 hp 10-12gpm pump for your well if it were mine. Cable will probably end up being 10 gauge, you’ll want submersible pump cable down the well, and UF cable in the trench. The threaded opening in your well cap is for conduit, it should be 1” electrical conduit, with a glued on terminal adapter at each end, add a plastic bushing at the bottom where the UF cable enters. If you’re a bit paranoid about losing the pump down the well, buy some pump rope and attach it to the pump, add an eyebolt to the top of the casing to tie it to. Do use a rubber torque arrestor on top of the pump, and add 6-10 cable guards about 10’ apart above that on the drop pipe. I’ve never had any real problems with the cable chafing with that setup. Don’t spread the diameter of the torque arrestor so much that it’s tight in the bore and tighten the hose clamps that hold it firmly, you don’t want it to get stuck down in the well. Good luck! There are a lot of details that matter in doing a good install even though the job is conceptually pretty simple.