Can I use an M2 SSD PCIe for my boot drive on Precision 7730?

There are a few ways to see what you currently have. Here's one guide.

Yes, your system can have a SATA drive and PCIe M.2 2280 drives, as you can see in Dell's specifications.

Here are some compatible drives from Crucial, though you're not limited to that brand.

You ask about "Gotchas"? Read the service guide, watch videos to get familiar with the process, assemble needed tools, be cautious and take your time. And always use a grounded wrist strap!

Follow the disassembly and reassembly steps in Dell's Service Manual:

Look on YouTube for videos showing disassembly of your model. But, be cautious with them, as some use steps or methods that are not appropriate. Use the videos to add to your understanding of what to do, but follow the steps in Dell's manual.

To switch from your current boot drive to a new drive, you may either use Macrium Reflect or Acronis to image the current system drive and restore to the new drive, or perform a clean install. I prefer a clean install of WIndows to the new drive. Note: remove the old drive before booting to the new Windows installation. After WIndows has booted from the new drive, you may replace the old drive to use as storage.

Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. I know many people do such work without this, but it's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.