I'm not exactly the expert on where to buy monitors, but I know that happcontrols.com at least has a 13" monitor:
http://happcontrols.com/monitors/4905480b.htmBut I bet you can do better than that with a little research. Hopefully one of my better informed byoac brethren will come in here with a better offer.
$100 for any cab in good shape is a pretty good deal. People have paid much less, and paid much more for them, but in the end it's up to you. Do you want that type of cabinet? If it's the right one for you at the time then buy it.
Don't worry bout the monitor part as there are several ways to fix it.
#1 You fix the existing monitor if it comes with one. People on here can help you fix pretty much anything.
#2 You buy a used monitor. You can probably find one from your local amusement vendor for a much lower price than a new one.
#3 You buy a new one. Pretty easy.
#4 You use a 13" PC monitor. These can be fit into a cocktail very easily and will be your cheapest option. You can usually find them on craigslist for close to nothing. If you plan on playing the original DK pcb then you will need to find a way to convert the signal to VGA. Or if you plan on using MAME then you will be fine.