If Johnson is part of the mix in 2016 there's a problem. His ceiling is as a #4 starter and Hammel already is that. Also everybody worries about trading him because of the money but he's paid like a regular season #3 and a fringy playoff rotation piece (I still think the injury bothered him in the second half more than anyone let on) at $10 million per. He's not overpaid and if you included him in a trade for the contact hitter you need, for whatever position you can fit him in, I think that's very do able. Also stop worrying so much about money. These guys are going to spend more than you think they are, but they're going to spend judiciously. Two starting pitchers, and two position players (for CF, maybe 2B and to fill leadoff and another contact hitter) is not a monumental task, not easy maybe but not impossible. These guys know what they have and what they don't.
The last 3 years I've hit their opening day payroll on the nose more or less. The only exception was when they failed to sign Tanaka but had they it would have been right inline with what I said. As for Johnson's ceiling, I've heard some suggest it's higher than that and frankly he has better stuff than Hendricks. MLB.com has his ceiling as a #2 though I'll concede that's probably too high. As for Hammel, while I agree he's not over paid, comparative to what you can likely acquire this FA period he probably is.
Regardless, I really fail to see the logic you're presenting with regard to acquiring two starters plus whatever they do in CF. I mean I understand you wanting to improve the rotation and in a perfect world I agree that makes sense but the logistics of it I can't see happening. As I said before, I imagine after a playoff run and a good year of tv ratings they may approach $150 mil opening day payroll. That's a $30 mil increase. They already have $79.5 mil committed to next season with $25 mil going to Lester, $14 mil to Montero, $9 to Hammel, $7.85 mil to Castro, $5.29 to Rizzo, $3.67 to Soler, $2.5 to Ross and $12 mil in dead money to Jackson/Conception. In terms of arbitration, you have Wood(Arb 3), Arreita(Arb 2), Strop(Arb 3), Cogs(Arb 2), Turner(Arb 1), Grimm(Arb 1) and Rondon(Arb 1).
Mlbtraderumors projects the following for those players
Clayton Richard (5.154) – $1.1MM
Chris Coghlan (5.148) – $3.9MM
Jonathan Herrera (5.101) – $1.1MM
Travis Wood (5.004) – $6.4MM
Pedro Strop (4.156) – $4.7MM
Jake Arrieta (4.145) – $10.6MM
Jacob Turner (3.033) – $1.0MM
Hector Rondon (3.000) – $3.6MM
Justin Grimm (2.170) – $1.0MM
I'm not 100% sure why cot's contracts doesn't have Richard and Herrera in their arbitration listings but I can't imagine Herrera is back anyways. You can make a case for non-tendering some of these guys but assuming they don't that's another $32.3 mil with Richards. Cot's contracts also suggest Cahill has an option year but I'm not sure if that is accurate given he was release and I assume that voids that particular deal since the cubs didn't pick him up off waivers. So, he might also have arbitration eligibility but meh. If he does it probably balances out anyone they non-tender.
If you add that to the committed money that's $111.8 mil leaving roughly $38.2 mil to play around with assuming my $150 projection is accurate. I have to imagine that Arrieta being re-signed will be a priority which depending on the structure of a deal could impact things. This front office has chosen to front load a lot of the bigger deals. However, I am going to ignore that for the moment since who knows what will happen. $38.2 mil basically leaves you with enough money for a decent starter and a decent CF. As I mentioned before, Span likely will be close to $15 mil and if they instead bring back Fowler you're probably talking close to $20. That basically leaves you around $15-20 mil depending on who you go with to sign a decent starter and even if you don't buy a FA, any decent pitcher you go for in a trade is likely in arbitration 2 or 3 and will likely be $7-12 mil.
My view is you get your 1 starter and see how Hammel/Hendricks play out until around midseason. Johnson being called up is possible. Edwards seeing time in the rotation is possible. Hell, you might even argue Turner has a strong spring and makes some waves. And assuming none of that works out, chances are you're still in great position to make a deadline move. Unless literally the perfect storm falls in your lap I don't see you being better off than that. And by that I mean unless you are able to sign Zimmermann or someone like him under market value and then make a trade for someone like Ross without giving away too much(i.e. not giving up Baez/Torres...etc).