Sealing windows without flashing or barrier

[Firsthomemaryland]

The previous owners closed in a porch to make it a three season room. We are trying to turn into an enclosed room. Eventually we'd like to replace the siding but in the meantime we wanted to start with the broken windows and didn't have money to do both at the same time. This would have been preferable because upon removing the windows and interior insulation it turns out there was no flashing around the windows or vapor barrier on the wall sheathing. The guy we had install the windows said there was no point trying to flash tape around them without being able to lap it over something and said a metal pan would do for now, insulate and caulk around the gaps and later when we do the siding we can add a j channel around the frames. The windows are in but I'm having second thoughts about insulating and caulking before knowing if we need to do some damage control first. Would appreciate any insight.

 

[joecaption]

I going to just make a guess since I'm not there to see this in person, so forgive me if I'm wrong on all the facts.
Someone installed these windows from the outside.
There the wrong style window, and size window, I'm judging this by what looks a built in sill, which would mean they where made for installing in a brick wall, and the big gaps on the sides.
They should have been replacement windows, that would have been just flat jambs with no bottom sill.
They removed the outside window stops, which should have been left in place to set the window frame against set in a bed of caulking.

 

[Firsthomemaryland]

I'm not sure about some of the other stuff but they were installed from inside, however there was no stop to begin with so I'm not sure it would have mattered. We asked for replacement windows so we didn't have to cut out the siding yet. I'm surprised to hear they aren't. I thought the main difference was the flange outside new windows. Now I'm thinking we just spray foam and run caulk between the frame and jamb for now and them add a nailing fin on when we replace the siding. It's not ideal but is there a better way now that they're up there?