The price difference between a j-pac and an i-pac is negligible. Wiring is the single suckiest part of building a cab (at least in my opinion) so if you can find one with good wiring that would probably be best. Plus, a machine with a working monitor (assuming it's working well) is a good price. You won't do any better without some luck.
Extra cost may come in when trying to get the PC to play on the arcade monitor, but there are ways even for regular video cards so long as they're newer. Otherwise an ArcadeVGA video card will run around $100.
Taking the $10 cab and adding a universal mount monitor would be the easiest method and best fit, but you'd have to buy the monitor. Making a 21" PC monitor fit in there could prove to be a royal pain in the butt, though.
If it were me...
Either take the $10 cab or buy the $100 cab and sell the monitor. Then buy a VGA 19" universal mount monitor. It shold plug straight back in where the original 19" came out. Then you're already almost halfway done.
P.S. Plus if you're looking for a trackball, I have a used Imperial 3" PS2 trackball for sale for $50 plus shipping. So it's an arcade trackball with a bnuilt-in encoder.