First, doublecheck your code. I don't know where you are, but in a lot of places there is a code addendum that allows only 5 square feet in area for a window that is on the first floor. In some places you also don't need egress windows in first floor bedrooms if the bedroom is within a certain distance of an exterior door - or in other places if it's within a certain distance of two different exterior doors that are a certain distance from each other.)
If you're going to stick with the 3'-0"wide double-hungs then yes, you're either going to have a fairly low sill height or a high header (the low sill height would be the more typical solution.)
Sometimes clients decide they'd rather go with the casement windows that are designed to look like double-hungs. It isn't a solution that I like, but it's one way to maintain consistency with other windows.

If your egress windows are above the first floor keep in mind that the sill height can not be lower than 36" abv. the finish floor without a guard rail in the window. Often such rails are put in for inspection and then taken out. Recomending guard rails to the client that can be taken out after inspection brings up liability issues. Ormolu's suggestion of casements is probably the best way to go.

You might want to look into that guard rail in front of the window comment before implementing it. My experience in residentail is that if the glass opening is closer than 18" to the floor or if the total glass area exceeds a certain value then you need tempered glass, not a guard rail.

We don't have that guardrail requirement here (but we do need tempered glass below 18" or within 3' horizontally of a door), though it's been a requirement in other locales I've worked in.
It's similar to the variations in egress size and such that I was writing about above in that there's a lot of variation in these egress requirements from place to place.
Do make sure to doublecheck all this stuff with your local code officials. In my state these egress window issues are all addendums to BOCA that you have to get directly from the state, so that you have to read each BOCA clause and then look up that number in the state's addendum and see if there's anything there differing from BOCA....

BOCA? You are using BOCA and not the new IBC?

The new IBC has been repeatedly delayed in my state. It won't be in use for at least another year.
When it is finally implemented there will be a similar state book of addendums/deletions/ammendments, as there is in most states currently.

you don't work near a coastline do you? if you do the new window criteria is extremely restrictive, and many window manufacturers have yet to pick up on the code change...