In short, you've already got the idea.
All a J-Pac or similar interface does is make connection between the PC, monitor and controlsvery simple and avoid the need for wire cutting and soldering etc. MAME can be set up to display many different refresh rates on a PC but you will likely need a way of outputting the picture at a low frequency for a genuine arcade monitors (someone here will be more technical). This can be done with an ArcadeVGA graphics card or by some software solution.
Once you have it all connected, you can run any kind of emulator/PC software in there you want and using a 'front-end' program you can hide the regular PC environment and do away with a keyboard and mouse for normal use, making it more like an arcade machine.