I wonder if AJ Pollock might be an outside the box candidate to spend some money on, planting Frazier firmly as the 2B starter from day one of the season? It's probably a decent toss up between him and Brantley for the best OF available outside of Harper, and both have injury concerns, but if we had Pollock and Marte at their best, we'd really be in good shape at the top of the order, especially if Frazier hits like we expect.
It's hard to project what kind of contracts will be given out and so on, and there are strong reasons to be skittish around a 31 year old who has had bad injuries and relies somewhat strongly on his speed, but these kinds of things matter less for the Pirates because the window is 2019/2020 and maybe 21. So with guys like Escobar and Pollock, if we have a year or two beyond that, we deal with that later on, and see what kind of rebuilding value we can get at that point for someone like Marte, Taillon, etc.
I fully expect to be convincing myself that Escobar is at least the most reasonable, but I think there are both strictly baseball and non-baseball arguments that are convincing to the effect that we need to throw some caution to the wind and spend in a vaguely moderately uncomfortable way (and I don't think we will actually do this, hence why I say I expect to be convincing myself that Escobar is most reasonable and what we can hope for).
Pollock
Marte
Frazier
Cervelli/Diaz/Dickerson
Escobar
Cervelli/Diaz/Dickerson
Bell
Moran/Other 3B/Newman/Other SS (depending on Escobar at SS/3B)
Pitcher
Seems like a much better lineup than almost anything else I can dream up. And once Polanco is healthy, it's not like any of the 4 OFs would be sure things, with in any case Pollock being some insurance for the years after. This gives you a fighting chance at a decently above average offense while still holding out some hopeful space for Bell and Moran to improve a bit, with Craig and Hayes on the horizon. The sundry of Reyes, Kramer, Newman, Osuna can be reasonable bench options, though that's not a strong bench necessarily.
I'd still want to see pitching bolstered by just getting a power lefty arm like Wilson, but otherwise I don't really think you need to touch it. We're set with Crick, Kela, and Vazquez, and still have RichRod as the swiss army knife. Losing Santana is a blow, but 4 extremely good pitchers remains a luxury, and the long-man roles + maybe one spot can be filled internally as cheaply as possible.
Would be helpful to get some baseline estimates for what the post-arbitration cap situation likely looks like, but just taking the salaries of S-Rod, Freese, Harrison, and Mercer and you have approximately 26 million. Free agent salaries are always inflated, but I can't really imagine Pollock + Escobar getting too much more than that combined. If I had to just guess blindly, given how many entry level contracts there are, we can probably make two mid-level signings like this and remain under 100 million.