It depends how down and dirty you want to get. Any 'kit' you buy will typically be assembled with low-grade chinesium.
Personal choice - I also got a set of moderately cheap brass pin punches to use as drifts because they're non-marring if I have to whale away on a resistant pin - they're useful for that.
- For general purpose, a good set of roll and pin punches and a suitable hammer.
- Non-marring hammer (not TOO heavy)
- Bench block
- Some high quality screwdriver bits or full screwdrivers in the correct sizes for gunsmithing (don't cheap out on these).
- For ARs, you'll probably want an armorer's wrench if you want to start moving castle nuts etc.
- A suitable gun vise or cradle, or a standard vise with a good set of soft jaw covers to avoid marring the gun. You need to have the gun solidly locked in place for many kinds of maintenance you might undertake.
- Pretty soon, if you're doing things like fitting optics or other torque-specific mechanisms, something like a Wheeler F.A.T. Wrench. (Edit: or as @GFB and others say below, something better)
- Cleaning kit
- Depending on what guns you have, a set of hex (Allen) wrenches. Again, don't cheap out.
As you go on, you'll assemble other bits and pieces like a can of Kroil, some light wet-and-dry abrasives. Maybe some Flitz, some needle-files and pliers. Don't be tempted by a Dremel. If you plan to disassemble a CZ 75 decocking mechanism, you'll need some paper bags, forceps and a heap of patience (Don't ask me how I know). A scalpel and some spare blades don't hurt. And some microfiber cloths. Some people have a set of dental picks and Q-tips on hand too. A couple of cheap parts holders from Harbor Freight are useful, but not essential (try and avoid magnetic).
Gunsmithing can be the most torque-sensitive engineering job you've ever done. Observe manufacturers' torque specs. Be aware that when you really lean on that vise handle, you CAN deform a barrel if you overdo it. Don't take a file to the gun's mechanism unless you really understand what you're doing.
Get hold of armorers guides where possible, or at least try and get schematics for the guns.
And again, personally, I have a magnifying glass desk lamp with a light, and a set of jeweler's loupes on hand because I'm old and my eyes ain't so good.
