Given what the front office/Bosio has done with starters I'm entirely unworried about 3-5 starters like you mention. Even if they have to find guys in FA they've proven to do that quite well. However, given the multitude of arms they've drafted over the past 3 years I find it fairly likely they will at the very least pull 1-2 guys out who can be 3-5 starters.
The real question becomes where you get your #1/2. They may already have a #2 in Arrieta. I hesitate to put him in the conversation for #1 even given his numbers. I think he's pitching like a #2 though as I've said else where I do think he's in line for some regression but he's consistently proven me wrong over his past 3-5 starts. Tseng is also someone who soon should be getting talk in that range. Then of course there's Edwards and Johnson as possible guys who could pitch to that level.
Who's the #1? Hard to say. Scherzer is a guy that should be available and they obviously should have the money to go after him. I think he makes a lot of sense and he's a good pitcher but I'm not sure I want him matching up against someone like Kershaw in the playoffs. I'd rather have him as a #2 even if you're paying #1 price. Honestly, I think the cubs will eventually trade for a TOR guy. Consider for a moment that most average systems have 3-5 prospects who may be in the top 100. The cubs have arguably 10+ who are in that range. They have the 8 obvious guys who were already in most of those rankings, Billy McKinney who arguably should get consideration, Schwarber who's going to make the list, Eloy Jimenez and Gleyber Torres who will soon start to get consideration, Tseng who should be top 100. You also have guys like Jeimer Candelario, Dan Vogelbach, and Christian Villanueva who will start to filter up lists once some of the big cubs names start landing in the majors.
Realistically, you could probably take one of the big 8 names and pair it with two of Vogelbach and Villanueva/Candelario and get a pretty decent pitcher especially if you're not making that trade at the deadline.