It's the wood trim on both sides of the window that would hold a sealed unit in place. The caulk would help a bit, but it's the wood trim that does the heavy lifting.
The best way to install a sealed unit that I know of is as follows:
1. you need to center the sealed unit in the opening to allow for expansion and contraction, and this is normally done by sitting the sealed unit on "setting blocks" which can be made of anything from wood to rubber. Typically, you only use two setting blocks under the sealed unit, but no one's going to bark at you if you use more. You typically only use setting blocks UNDER the sealed unit, not at the sides or on top.
2. Now, use GLAZING TAPE on the outside perimeter of the sealed unit. Glazing tape is butyl tape that comes in different thicknesses, and is available at any place that sells weather stripping or parts for windows and doors, or at your local hardware store. Some people put the glazing tape on the window perimeter, others put it on the exterior wood molding before putting the window in place, and others use glazing tape on BOTH sides of the sealed unit. There's no real right or wrong way on this aspect of the work. The job of the glazing tape is to seal the joint between the exterior wood molding and the perimeter of the window, while still allowing the glass to expand and contract with seasonal changes in temperature. You can put the glazing tape on the exterior molding first, and then fit the window into place, or you can put the glazing tape on the sealed unit and then center it in the opening. The glazing tape will be sticky on both sides, so it'll have a paper backing on one side that's pulled off immediately before setting the window in place against the exterior wood moldings. Perhaps someone who's done a lot of this kind of work (I haven't) will recommend the best procedure to follow when installing the glazing tape and the sealed unit.
3. Once the sealed unit is in place, nail your interior wood moldings in place. I find when setting the nails, it's best to put a small piece of wire insulation over the tip of the nail set to prevent it from slipping off the finishing nail head and punching a hole in the wood molding.
4. Caulk between the exterior wood moldings and the sealed unit with a synthetic rubber construction sealant like Kop-R-Lastic. If you can't find Kop-R-Lastic in your area, phone your local window companies and see what they use. Caulking windows is an important part of installing windows, so window companies won't take the decision on what caulk to use lightly. They will use the best caulk available in your area.
(I won't use ANY caulk except Kop-R-Lastic on a window or door. The reason why is that it sticks to common substrates as well as any caulk needs to. But, it's COHESIVE strength is higher than it's ADHESIVE strength, which means it sticks to itself even better than it sticks to the substrate. That means that when you want to remove this caulk, you simply get one end started, and it pulls off the window like a rubber rope. That makes replacing the caulk a quick and easy job, and that makes proper maintenance of your sealed unit a job you don't dread, and that ensures happy and healthy windows.
USE Products Ltd.
Kop-R-Lastic is made in Canada by the U. S. E. Hickson Company, who also makes Stone Mason products. If you can't find Kop-R-Lastic, then see if your local Home Depot sells Stone Mason Gutter and Siding Sealant (in clear or white) in the same aisle as they sell evestroughs. It's exactly the same stuff as Kop-R-Lastic being sold under a different name. Also, the Henrys company of the US has purchased U. S. E. Hickson of Canada, and so Henrys may now be marketing Kop-R-Lastic under a different name in the USA.
5. Prime the windows with an exterior oil based primer. Then paint with a flat exterior oil based paint, and run the paint up about 1/8 of an inch (or more) onto the glass.
6. Now, take a look at the roof overhang over the window. It's a known fact that the shorter the overhang protecting the window from Sun and rain, the more problems you're likely to have with the window. If you've got less than a 2 foot overhand, and you want to do a Cadillac job, read on... (My building has a flat roof, so I don't have any overhang whatsoever, and that's why I need to protect my windows more than a homeowner would.)
7. Look in your yellow pages under Adhesives and buy some weather proof double sided tape, and some weather proof single sided tape.
8. Phone around to the places listed in your yellow pages under windows and doors to see who'll put a 15 degree bend in some 4 inch wide strips of white PVC for you. The bend should be lengthwise along each piece and located the same distance from one edge of the PVC as your double sided tape is wide. The idea here is to make a little PVC "roof" to protect the bottom exterior molding of your window from both the Sun and the rain. (You will need to cut the ends of the PVC strips to fit the sides of the window, but this can easily be done with "crash scizzors" that you can buy for $2 or $3 from any place that sells safety supplies.) Not only will this vastly extend the life of the window installation, it'll make the installation almost maintenance free because it's the Sun and rain that causes paint to deteriorate on wood, and wood moldings to deteriorate. Paint that stays in the shade on dry wood lasts darn near forever. You stick double sided tape to the PVC strips and stick them to your window about 1/4 inch above the exterior wood molding. Then you take your single sided tape and put that over the top edge where the double sided tape is to prevent water from sitting on top of the edge of the double sided tape. And, by providing a 1/4 inch gap under the PVC "roof", any water that does find it's way under that roof can evaporate. If a crack develops in your caulking or wood molding, then water that gets in there will be slow to evaporate, and that can lead to wood rot.
Also, with the PVC roof 1/4 inch above the exterior wood molding, you can check the condition of the paint on the wood molding from inside the house to see if it's starting to peel anywhere (or from the outside with a small mirror. Paint peeling on wood most often indicates that the wood is getting wet. You don't want that. Dry wood is happy wood.
The reason you want to make your roof out of plastic is so that if you ever have to remove it, it'll keep it's original shape (unless you bend it way out of shape). Metal would work, but it'd bend if you try to remove it.
Finally, I own a small apartment block, and I've done this on each of the sealed units in the back stair well of my building. I can vouch for Scapa ASI / 77G - PE as an exterior rated double sided tape. I've very happy with it's performance outdoors. I can't say the same for the exterior single sided tape I've used, which is Scotch 4811 YT 0 4271. The latter peels off by itself.
Hope this helps.