Mercury surface is very hot (430C). Additionally, the Sun radiation as you approach Mercury with your mining mission could cook your ship's electronics and personnel. So, you need to find a way to approach Mercury. The best way is to approach Mercury at such an angle, as to be in its shadow most of the time. Unfortunately, Mercury is small, so I think you could only benefit from its shadow for the last few million of km (I didn't calculate how many) of your approach.
This brings me to the first issue. You need to make sure your ship electronics is resistant to heat. That leaves out a lot of nifty discoveries in microelectronics that make possible today's computing. Your ship electronics have to be simple and extremely robust. Mariner 10 mission approached Mercury, so that much is possible. But, you cannot bring people to Mercury, unless you develop a cooling system and radiation shielding to protect your ship. If you don't, you have to develop materials and new electronics that enable high density information processing and storage at up to 500C, at least.
Then, you have to develop the AI for your ship and robots that would enable you to complete the mission and handle unpredictable situations. I'm not sure if that is doable now, but machine learning has made lots of progress and it is conceivable.
Once you get to Mercury, better land on the dark side (Mercury is tidally locked). You need to build your basis, and bring enough material to be able to start the mining operation. Your space ships can only bring enough material if they have nuclear reactors. If you don't have the technology, better hope you have all that you need to build on the planet. In that case, you need to do a lot of exploration and that might prolong forever your mission.
To dig at 200 miles, I think is only possible with atomic blasts. Assume your hole is similar to those for diamond mining, you should dig an area of at least 100m radius, to have a chance to get to the bounty. That's about 1 billion cubic meters of material, assuming your hole has vertical walls. But, if you dislodge the dirt with atomic blasts, it will most likely be an inverted cone. In that case, assuming a 15 degrees for the cone's angle, which I think is conservative, but possible, the material you need to extract is in the range of ten thousand billion cubic meters. The other thing is that Mercury is quite dense ($5.4g/cm^3$) so you need to break the rock somehow.
You can also do mining on the hot side of the planet. You can use solar concentrators to melt and then solidify the walls of your hole to stabilize them after you extracted the rock.
Looking at the numbers, I think the biggest challenge is getting the rock out. For that, you cannot bring enough equipment from Earth, because Earth has a limited supply of metals. I think you should find enough metal on Mercury, but you should develop an planet-wide chemical industry to separate it and make more machines. That is likely to take lifetimes to finish. One way to do it, would be to make your machines self-replicating.