Basically you need a few things. You need good quality bread and heat application. I know you don't want to cook but all bread is horrible when you don't at least toast it. Get one of those lean mean grilling machines. They can sit on your counter and there's basically no cleanup.
Even a small one works fine and you're not really cooking you're just assembling the sandwich and closing the lid and letting it toast.
Second thing I suggest is this: https://gamefaqs.gamespot.com/a/forum/4/46168ea2.jpg
That will help you assemble quick easy fast vegetables/fruits very finely sliced. You might still have to cut a tomato, but remember the trick with all of these is to make the slices as thin as possible.
Last find you a recipe for a panini that sounds good. Heck I made an amazing ham, apple and gruyere cheese sandwich topped with mayo that was mixed with whole grain mustard and fresh thyme and that blew my socks off.
If you want another recipe that's actually cheap I know an open-face tuna recipe that will blow your socks off