Window Sill

[DewWars]

We have a 1920 house we purchased for us. I've removed all the crumbling plaster including lathe. Well used the old strips of lathe to shim out the exterior studs so the drywall fits flush on the window frames. Surprisingly the frames are all square and free of any rot. The same with the exterior siding and trim, except an area around the side door with terminate damage but that has all been fixed.
Enough of the rambling! The next phase after rewiring, insulation and drywall is putting in new replacement window inserts. The plan is to put them in from the inside by leaving the exterior sash stops. I'm sure there is somethings I'm missing but I'm here to learn from the pros and get everything ordered correctly ahead of time.
The main question I have right now deals with the window sill. Our sills are all flat and every window place I go to say they have never seen flat sills they are all angled. I'm still hoping to use the current interior trim. Am I supposed to have them angled or will flat suffice?
I'll try adding pictures...
The exterior picture, where the dirt circle is, is the window frame/bottom sill.

Exterior- https://ibb.co/fYgWQM5

[Delaware Mike]

From what I see in that picture you should be fine. Inserts have their own sill pitch of about 5-degrees for water drain off built in with a flat bottom extrusion unless it's some sort of pocket sill system. Some really old wood windows do not have a 3 5/16" to 3 7/16" pocket between the exterior and interior stops for the 3.25" insert to fit the inserts into, thus in your case, you would simply need to rip down the interior stops a little.

[toddinmn]

It looks like someone added a 3/4” board on top of the actual sill .The sill appears to have a slope. I would
Most likely remove the 3/4 board.

[DewWars]

It looks like someone added a 3/4” board on top of the actual sill .The sill appears to have a slope. I would
Most likely remove the 3/4 board.

That 3/4" is the window frame, the thick board underneath was installed on the exterior. Might just be the angle I took the picture.

[toddinmn]

I can see what’s going on from the angle. . Perhaps it’s something I am not familiar with or maybe the windows have been replaced since the house was built. It would definitely be abnormal for me if the windows are original to the house and even abnormal if they were a replacement window after the that. I am not in position to advise at this point.

[TheWindowNerd]

You will be fine with a standard square bottomed insert window such as most vinyl insert windows have.
A problem could arrise if you use a window that has a tapered bottom on the bottom of the sill extrusion. Such windows would include Pella P Fit, Marvin DH Essential or Elevate, AW 400 insert DH insert windows

wayne theWindowNerd