According to my butcher - real beef tips (in this context) are trimmings from the tenderloin and sirloin primal when broken down to the various cuts. For example, on a tenderloin it is the tapered end too small in diameter to be used for fillet mignon. Trimmings from anything else is stew meat or hamburger.
Yes, you can use cubes of sirloin steak or tenderloin. Think of it along the lines of buying a chuck roast and cutting it into cubes instead of buying "stew meat" scraps. Since you live in Austin you might call the butchers at Central Market and see if you can get a deal on some "uncubed" sirloin or tenderloin tips and cube them up yourself.
I prefer mine made without tomatoes so, IMHO, any tomato sauce or paste would be too much.
Personally, I don't butter my noodles. I feel the butter makes the noodles too slick for the sauce to stick to. But, if you want to butter your noodles then do it.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain