Plan away, but I don't expect a major FA signing unless one of the pitchers finds out the market isn't there and is willing to sign a 3 year deal for reasonable (i.e. market with a premium) money. I don't think Klentak trusts that anyone available is going to be more than a back of the rotation guy by 2020, and they really don't care about 2018 enough to grossly overspend.
However, I also expect that the market was set by the 2015 FA signing orgy, and FAs who think they're going to get huge contracts are in for disappointment - Darvish might get a gross overpay by one or two "fools," but most teams have been burnt enough to be wary of guys without track records (what you did five years ago is meaningless, it's what you've done the last couple years that's the best predictor of future success).
Reality is that most teams now sign their best FAs before they hit the market, and any FA that hits the market is "buyer beware" (since the team that knows them best won't pay to keep them). If the Phillies want to build a solid pitching staff, they're going to have develop most of it in house and trade for the rest, free agents at best will be "hamburger helper."
See Cleveland:
Kluber, 4th rd 2007 (SD), traded in 3 way deal for Jay Westbrook, 32 year old SP who put up 92 starts, 1.3 WAR (BR)
Carrasco, Cliff Lee 2009 trade
Bauer, #3-2011 (AZ), 3 way deal, Cleveland basically gave up Shin-Soo Choo, 29 year old LH OF, 3-4 WAR player, a LOOGY and a couple 2nd tier prospects
Tomlin, 19th rd 2006, one good season at 26, bounced back at 29-30 years old, 1-2 WAR back of the rotation starter
Cleavinger, 4th rd 2011 (CAA), traded in 2014 for Vinnie Pestano, 29 year old middle RP,
Salazar, LA signing, TJ 2010, middle rotation guy - "He was rail thin and throwing about 87 mph," gave him a $200K bonus
Now it's doubtful Klentak's gonna have this kind of trade success, but it does show you the best way to build a rotation.
How about the Boston pitching staff, big spenders right?
Sale - 13th-2010 (ChiSox), traded after 2016 season for two top prospects Kopech and Moncada, and two second tier prospects
Porcello - 27th-2007 (Det), traded after 2014 season for Cespedes (1 yr from FA), Alex Wilson, 28 yr old RP, and a second tier prospect
Pomeranz, 5th-2010 (Cle), traded to Colorado 2011, Oakland 2013, to SD 2015, to Boston 2016 for Anderson Espinoza, BA rank #19 at the time
Rodriquez, signed by Balt, traded in 2014 for Andrew Miller, 29 year old RP, one year as a closer, basically an 8th inning RP
Fister, 7th rd-2006 (Sea), signed off waivers
Price, signed in 2016, 7 years/$217 million, 30 years old, 4.8 WAR first two seasons, only started 11 games this year
Ok, the Yankee staff was bought, right?
Severino, 2011 LA signing, $225K, 91 MPH
Tanaka, signed out of Japan in 2014, 7 years/$155M, 11.8 WAR first three seasons, however, really fell off this season
Montgomery, 4th rd-2014 out of college
Sabathia, signed in 2009, 9 yr/$206M at 28 years old, 21.8 WAR first four years of his contract, 6.5 WAR from age 32-36
Pineda, signed by Sea, traded in 2012 for Jesus Montero and Hector Noesi.
Gray, traded in 2017 for two top prospects Kaprielian and Mateo and a throw-in.
It seems the best use of money seems to be paying some really good scouts to find hidden gems in other teams' farm systems!