How tough is it for a fairly skilled amateur to change out a Marlin guide gun barrel.

Does it make more sense to go to a gunsmith as opposed to buying go/no-go gauges and maybe a vise?

I would recommend getting the vice. They are nice to have and not that exspensive (Mine is aluminum and fits into the jaws of my vice). As for head space that is something anyone can do, it closes on the go, and doesn't close on the no go. The real problems arise if the barrel doesn't line up properly, ie the sights end up canted to the side instead of the top. It may then require the threads to be cut back to line it up right, which is a smithing job. If you try it make sure you pad the pipe wrench with lead pads to turn the receiver otherwise you way ruin it.

Thanks for the encouragement.

I'm tempted, but I think I'll let a gunsmith do it.
Others have told me that the Marlin receiver can be torqued out of shape if the wrong tools are used.

If I was going to be changing out barrels often, then I'd buy the vise and the marlin receiver wrench. But once I do this swap, I'll be done.