Reddit mentions of Bondo Body Repair Kit, 00312, Everything You Need to Get Started, 1 Kit

I run this instead of a Dremel brand tool. I have had a couple Dremels die on me over the years, so I tried this one and it has lasted 4 years so far! I also got a lot of my bits from harbor freight. I use the cuttoff wheels (ALWAYS WEAR FACE PROTECTION WITH THESE), Sanding Drums, and grinding stones the most. Use the cutoff for cutting out large swaths of material, sanding drums for tighter areas, and grinding stones to debur and smooth it all out. Most work that needs a dremel can be done with hand tools for better control. Coping saw, files, and flush cutters can go a long way. A lot of the "pros" don't use Dremels because they can jump out of control and hit a part of the blaster you planned on leaving stock and leaving a huge gash.

As for soldering Irons, I've heard wonderful things about this model, but cheaper models will work fine. TBH, I'd grab something in the $10 range and see if he sticks with the hobby. Maybe for christmas you can get him a nicer one if he's still going at it. I use a super duper old school Weller soldering station, so really anything will work. The issue you'll get with the cheaper soldering irons is the tips will tend to not last long and the heating element might crap out after a few months. Don't go cheaper than $10 here as some of them are meant for wood engraving, not soldering and wont get hot enough to melt solder. Any size solder is fine. I personally use lead/tin solder and just avoid breathing fumes as well as wash my hands when I'm done modding. Lead solder is MUCH easier to work with and requires less heat. Look for "Rosin Core solder" as it's easier to work with as it has flux in the center of the solder.

Good flush cutters, hobby knife, wire strippers, heatshrink, wire, loctite, etc are all valuable modding supplies. We sell some on our shop (shameless plug). I would recommend for plastic to plastic bonds he use Devcon Plastic Welder (make sure it contains Methyl Methacrylate as this acts as a solvent weld to bond the plastics together chemically instead of a weak adhesive grip like putty). Putty should be used as a filler material. I use two different putties. Free form Air for large amounts of gap fill, it is 6 times less dense than normal putty so it wont make the blaster weight 8 pounds when done. It is VERY easy to sand, but sometimes requires a thin layer of bondo over top of it to leave a perfect finish. I also use Apoxie Sculpt which is a direct replacement for normal putty for gap filling purposes. It's MUCH easier to sand than JB Weld putty, and leaves a nice smooth finish when sanded down.

Speaking of sandpaper, get various grades of sandpaper from 100 down to 600 grit (I go as far as 1000 if I want a gloss sheen on something).

Other than all of that, the tools I have on my bench that I would hate to mod without now are: Helping hands (for soldering), various different sizes of needle nose pliers, numerous sizes of small to medium screwdrivers, silicone lubricant, hot glue (for securing wires inside the shell), Electric Screwdriver, and various tweezers.