Depends on how much you're willing to spend and what you want to do with it.
For typical flywheel mods, a 30W or 40W (watts) Radio Shack pencil style iron will suffice, but barely. It'll be good enough for splicing wires, attaching connectors and soldering wires to motors.
However, if you want to build your own battery packs, you'll want an iron with more power. I'd recommend at least 60W.
If you want to use the soldering iron for other tasks, such as sensitive electronics boards, you'll want something lower powered, such as 20W.
So what am I getting at? A more flexible soldering iron will have an adjustable temperature; it'll also cost more.
I personally use the Hakko FX888D, but you don't need to spend that much, although you'll need to spend close to that for a good iron. Another good brand is Weller, such as the WLC100, although I think 40W isn't enough power to safely build battery packs. As long as you won't be building battery packs, I think the WLC100 will be perfect for typical Nerf flywheel mods. You could get the WES51, but that's pretty much the cost of the Hakko.
There are cheaper options with adjustable temperature settings and goes up to 60W: The TrakPower TK-950. I believe it's a rebranded Hakko, but I'm not positive.
As for tips, whatever comes with most of the irons I recommened are good enough for anything you'll need to do with flywheel Nerf mods. I bought a smaller and broader (chisel) tip when I got my Hakko, but I've never used them yet and I've worked on things from Game Boys to battery packs to Graphing calculators to my Nerf Stryfe.
EDIT: Here's an option:http://www.hobbyking...Warehouse_.html
I'm betting it's a rebranded version of a name brand iron, such as Hakko. However, for such a cheap price, I'd be wary. But then again, for such a cheap price, it might be worth trying?
Edited by shandsgator8, 25 December 2015 - 05:19 PM.