You might want to consider a couple of options:
Build a couple (decide if you want empty cabs, or if you want to put a PC and some sort of standard controls in it or whatever). Once you have a couple built, try and sell them locally. You can either do eBay (Local pickup only), or maybe advertise thru Craig's List, or some other classifieds. Put up a website with some pics. Nothing fancy. Go to an arcade, or a place like Dave & Buster's and talk to some customers. See if they would be interested in buying one of your cabs (just don't be obvious about it or they might kick you out for soliciting). Bring pics & prices w/ you.
Anyway, the point is to build a couple and sell them locally to see if you can even sell them, and how much people are willing to pay for them. Maybe have 2 prices:
1 for an empty cab & empty control panel
1 for a cab plus control panel (w/ buttons & sticks) & maybe a cheap PC & monitor. A "barebones" cab, so to speak. Put PC-Dos on it (it's free) and maybe load up ArcadeOS, and leave the ROMs to them to get (or whatever).
See what kind of bites you get.
If you can sell them pretty easily and make a good enough profit, maybe you can take some custom orders (different control panel options/colors, etc.)
This way you can see how much labor is involved vs the profit, and you really have a minimal investment in money & time compared to trying to go national immediately.
If you have easy access to CNC machines, then making cabs & panels is a breeze, and you can duplicate stuff perfectly every time. If not you gotta do it by hand and that's a lot of labor.
A friend of mine was interested in this as well. I told him that I thought that the way to go was to make a port-a-mame. Build a control panel & very deep box. Mount a motherboard (mini-itx, or just a really small, 2 slot ATX MB) inside the control panel box. Use a cheap video card that has s-video out (Xpert@play 98 is one). Run PC-dos & possibly arcadeOS. set everything up on the box (PC-wise) the way you want it (minus the roms) and ghost image the hard drive. This cuts down the install times, etc. You could also install Linux on it and run the mame client for linux if you were so inclined (and your customers are a little familiar w/ Linux - most people will just power it off w/o shutting it down properly which can lead to more problems in *nix than in windows)
See if you can get $500 for it. This thing would be portable, and can hook up to both a TV and/or a VGA monitor. They can put it on the floor in front of the TV like you would an Xbox. The price difference is the fact that it is also a PC. You could put all sorts of additional features on there like some sort of an arcade jukebox and they customer can use the ethernet port from the MB (that you mounted into the box w/ easy access to the RJ-45 plug), and connect it to a media server, or whatever. If you have a CD-Rom drive installed (reccomended) then they can pop in a CD or an MP3 loaded CD & play it (speakers sold extra).
It's pricey, but it can do more than just MAME, AND it's portable! They can take it over to a friend's house, which you can't do w/ a MAME cab!
Not only that, with other emulators, they can play Atari 2600 games, NES, SNES, N64, Sega Genesis, etc., etc. all on the same device.
This may all seem obvious to us, but to those who know nothing about MAME and the other emulators out there, they would simply freak at seeing something like this! Think about it. You've never heard of MAME, or knew that there were emulators out there. Then someone shows you this box that looks like an arcade control panel. They hook it up, and not only can it play all of those classic games that you remember, but it can play NES games, Atari games, everything. I know if I saw one I would freak out! $500 may be a lot, but when you sell it on the fact that it is also a computer that can do other stuff, the cost does not seem like so much. If I knew nothing about MAME and someone showed me this I would probably start saving my pennies immediately for it!
I actually plan on building one like this myself (for mayself) at some point. In the meantime, I have 2 other plans that I am going to be working on first! When I finally make one (whenever that is) I hope to be able to build a 2nd one as cheaply as possible so I can give it to my cousin's kids for X-Mas (not this coming, but the one after). They are 9,7, & 5. Perfect age to play this stuff, though I do imagine my cousin playing it more than his kids! :-)
Brain21