I think both moves do make sense. Marte has two years, both at slightly over $10 million; as a ~3.0 war player, he’d be a great OF upgrade in tandem with Donaldson. Sans Donaldson (or the Rendon pipedream), I think an OF (or SS) upgrade needs to be loftier (then try to eek 2.0-3.0 war out of a 3B platoon).
I think the people talking about a move that substantial are dreaming a little too much in regards to the outfield. First, those two would add around ~35 million in 2020 payroll, carrying our ultimate number well over 170 million dollars if we didn't shed any other contracts. Even if we shed Ender's contract, it'd still be well over 160 million. That ain't happening. It would take a few other contract moving moves in order for a Marte/Donaldson sequence to be even remotely feasible. Secondly, I think the front office would be fine rolling with Duvall, Ender, Markakis, and Acuna in the outfield in 2019. They wouldn't be ecstatic, but they wouldn't be suicidal either, especially if they effectively address 3rd base. But, I do think that they would like to add a right handed bat to the outfield as the weak side of a platoon with Nick and Ender, but an acquisition like that is going to be far less substantial than Marte. Third, if for some reason they decide to go after a big bat like Ozuna or Marte, it would almost HAVE to be in tandem with a lesser-than-Donaldson player at 3rd base. Ideally, in that situation we could work out a trade for a guy like Escobar whose extremely cheap contract MAY allow for a bat like that in the outfield. But, once again, I think that specific circumstance is fairly unlikely.
We are currently sitting on 138 million in payroll. The absolute ceiling I see for our OD payroll is about 155 million, and that seems very optimistic. So in the best possible scenario we have 17 million left to spend. Most, if not all, of that is going to have to be directed at either outfield or 3rd base, not both (unless the aforementioned miracle trade is in the works). And out of those two positions, 3rd base is easily our more pressing need. So I rank these remaining options in order of preference, not likelihood and assuming a 155 million dollar OD payroll:
1. Trade for Escobar (or similar player), Trade for Marte or sign Ozuna (may have to move contract in case of the latter)
2. Sign Donaldson, move contract for payroll, sign extremely cheap RHH outfielder or stand pat
3. Trade for Escobar (or similar player), Sign 2nd tier or reclamation outfielder (Puig, Garcia, Pillar, Santana, etc.)
4. Sign 2nd tier (or reclamation) 3rd baseman (Shaw, Gyorko, Cabrera), do same for Outfield (Puig, Garcia, Pillar, Santana), possibly leaves room for small bench signing as well.
There may be a few other options that people could work out, but these are the ones that I think are the most likely. If we don't have the payroll capacity to hit 155 million on OD, then things become far more complicated. In that case, these are the options I see:
1. Move a 10+ million dollar contract, then revert back to the 4 aforementioned options.
2. Trade for Escobar (or similar), stand pat in outfield, sign replacement level player, or somehow trade for cheap production.
3. Sign ~2 cheap utility options or reclamation projects to mix and match on offense and hope you can approximate last year's production through platooning (Zobrist, Cabrera, Sogard, Frazier, Flores, Miller, Walker, Gyorko, Shaw, Beckham, Santana, Pillar, Maybin, Puig) - Some of those options may be priced out, but most were <2 WAR last year.
The more I look at our current situation, the more I'm worried about effectively filling this offense out in 2020. AA is going to have to get pretty creative to figure it out.