If you can do the prep work at the shop or wherever you're getting the engine replaced, then it should really take only a few hours. Really, engine replacement is NOT that difficult. You don't have to even remove the A/C to do it. Just the rad and the intercooler (if you're careful...)

The toughest part is getting all the accessories off of the block. Once you've done it a few times it's a breeze, but the first time plan on several hours. At least you don't have to worry about manifold studs and whatnot, because you'll have an engine sitting there presumably with either studs in it, or clean.

Once you pull the intake manifold aside (you've removed the intake hoses and whatnot) you can pretty much unbolt the bellhousing, the engine mounts, undo the torque converter bolts, and yank the engine.

I would budget $500 or so for engine removal/replacement. It might be more, depending on what kind of prep work you can do. The $400 is for the basic R&R, no frills, some prep work done. If you can't do all the stuff I mentioned above, it would be $500 or so. Maybe even a little more. But prices for that kind of thing vary greatly. Some places go right by the book, and others have different price structures for removing and replacing big components like engines and transmissions. One guy does a simple trans exchange (not including parts) for $200, and the other guy does the same job for $600. And both are really a fair price, but it depends on what you use as the judge of that.

Some shops will tell you you have to have the A/C discharged
Now, don't take this the wrong way, but usually when someone starts asking about doing their own prep work and whatnot, there is a lot of room for things to go wrong, and as such, most mechanics shy away from that kind of thing. Usually you can deliever a whole car, or don't deliver it. Unless you're friends with your mechanic and know what they expect (and they know what you expect) just shop for the price and leave it at that. Otherwise you'll feel that you did more work than they're giving you credit for, and they'll feel that they had to re-do half of your work and should charge appropriately.

Trust me, this is just how it works. Unless you can almost read the mind of the guy who will be doing the work, you might as well forget about doing the prep of the car. He'll feel that he has to re-do every single thing you've done, and he won't know where half the parts are, where the stuff goes, etc. No wrench likes to start a job that's halfway done.

Now, as for the enigne, and getting the NEW one ready, I'd suggest finding a place where you can re-seal the whole thing. New rear main (and rear main flange gasket too). Front 3 seals, new valve cover gasket, intake manifold gasket, throttle body gasket, and whatever else you can see that might need work. Distributor O-rings maybe, and then move on to the timing belt, water pump, and so on. It's so easy when the thing is on a stand.

Now, having mentioned $500, you have a place to start. I think you should be able to get it done for that price, and probably a little less.
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1992 940 wagon, low miles
as well as others.