You would spend a lot of money on a kit that could cover all gunsmithing.Get a brass and a steel hammer and a really nice set of screwdrivers and a nice vice and you will have a start.
1. Good sets of precision drivers not made in China. You'll need Phillip, Slotted, Torx and Hex. Don't skimp on the precision drivers. Get Brands like Wera, Wiha, Bonhus, Snap on....
2. Vises - A bench vise and one of these articulated:
It's is a beautiful thing for holding handgun frames. Tear the rubber vacuum base off (it's worthless), drill some hole in the base and screw your own weighted base to it.
3. Brass Hammer and a BFH.
4. A set of Real Roll Pin Punches and a set of Drive Pin Punches.
5. A flex head light. This Greenlee from Lowes is the ****:
Seriously, some of the most effective tools a gunsmith can have is good quality and proper cleaning tools. Unless you specify what kind of gunsmithing you're wanting to do then that's the most universal that will solve a lot of issues.
You want tools that won't mark up steel. Think brass, bronze, and nylon/plastic. Good solvents and use them as per instructions (sweets 762, butches bore shine, hoppes), lubricants and grease, and good lighting. Start cracking open your guns and the tooling that will make each job easier will present themselves. Most general smiths have over $30k invested in general tools and holders specific to different guns. They don't come in a kit.
Maybe I should clarify. I am NOT looking to mod any of my handguns. What I am lookin for are appropriate universal type tools needed to properly detail strip a handgun should a part need replacing or detail cleaning. Should I want to mod a gun, I would bring it to a professional smith. So, no, I was not kidding.