Back to the point of a swap... How difficult would it be into an older (SF) model? No more work than any other swap?
Well... Define 'no more work than any other swap'. I won't claim to have extensive gas-to-diesel swap experience, but from the couple I've been peripherally involved with it's not just a case of pull one engine and install another, and in both cases that was with vehicles designed from the get-go to have both gas and diesel variants. So, thinking out loud:
In this case, I'd expect that you'd need the engine and ECU for starters and you may as well grab the wiring harnesses for both as well. From there, you've got a good-sized wiring compatibility project to undertake between them and the existing wiring harnesses; this is in addition to figuring out what to do for instruments and idiot lights because they almost never match up to the signals the new ECU/sensors are sending them (and you now have new lights or displays that need to work for things like glow plug activity and water-in-fuel warnings).
The fuel filler and tank may require replacement as well as all of the fuel lines up to the injector pump, though the first two at least likely fall into the 'maybe' category. You'll need a water filter inline with the fuel lines as well, so will need somewhere to stash that, though it may be part of the diesel fuel filter (common practice these days).
Physical fitment of the engine: diesel engine mounts are typically different (heavier-duty) to their gasoline counterparts, so assume those will need to be changed. If the engine mounting location doesn't match the EJ20/25's exactly, bolting up the transaxle will be an interesting exercise. Getting the turbo and particulate filter (both located next to and in front of the sump) on the EE20 to slot in at the front of the engine bay could be a potential nightmare. And that all assumes that its physical dimensions (which I haven't been able to dig up) are even within the EJ25's to begin with.
There are 8730251 little things that aren't being considered here, either - driveline components, etc. are all now being subjected to nearly 60% more peak torque than they were when the EJ25 was in there, brakes need to be swapped over from the donor since there's no longer any manifold vacuum to drive them, cooling system differences, and a whole bunch of other things.
I don't see the savings in fuel cost ever making up for the swap price, and it could only be in a state with no emissions testing. (the only good thing about FL)
Yeah, the distance you'd have to drive to make up the cost of the swap means that you'd pretty much never hit the break-even point in normal usage. If you're doing astronomical annual mileages it can make sense, though.
There's a thread here
in which a guy is swapping an EE20 into his Vanagon. While not completely related to putting one in a Forester, it should give some idea of the sorts of things you can run into.