I'm sorry to hear of your loss.
What you're doing is very difficult.

I'm tempted to suggest one of my favorites, FF Avance, because it's a playful but ambivalent design, full of intelligence but still uncertain of what it will become. Also, it has no overly-thin parts which can get lost in engraving. Lastly, it was designed by somebody who also died too young.

http://new.myfonts.com/fonts/fontfont/ff-avance/

hhp

Another option for your selection
http://new.myfonts.com/fonts/fw-smith-hands/english-engravers-roman/

A lot depends on how smooth the stone will be, and how patterned.
If the surface is polished and there is no pattern, then a finely detailed face will hold up.
So the choice of typeface (and size and layout) should work in concert with the choice of stone.

As you intend to deviate from off-the-shelf template options, it may be advisable to engage the services of a professional designer.

What would she have thought of the image you suggest? Depending on the execution there is the possibility it could become overly sentimental.

A contemporary* type in the classic "stone-chiselled" mode is Carter Sans—very robust and un-girly. It would hold up well, even on dark, coarse stone.

*Cousin to the Internet face Verdana, by the same designer.

> What would she have thought of the image you suggest? Depending on the execution there is the possibility it could become overly sentimental.>

I appreciate and understand your concern. I am not into any sentimentality either, and I will be very careful not to allow that.

> As you intend to deviate from off-the-shelf template options, it may be advisable to engage the services of a professional designer.>

You are right again. But I have no idea how even to start looking for one, where to find them. The monument companies are awful designwise.

I like all three suggested typefaces and will consider using one of them. I am very grateful for all suggestions.

Possibly a local graphic designer, they tend to be generalists.
Look for one who has done work with special printing processes such as embossing, foil stamping, spot colors, unusual paper stocks—or "environmental" graphics i.e. signage. This would indicate that they would have some experience relevant to the tactile requirements of gravestones, even if they have never designed one.

Alternatively, an architect?

If you chose someone local, you can look at stone samples together and discuss with the mason/stonecutter.

You may have to go further afield for a specialist in stone carving.
From the top: http://www.johnstevensshop.com/

Getting the assistance of a designer is an excellent idea, but most graphic designers will have zero experience with this kind of project.

I think Nick's idea of talking to local masons/stonecutters might be your best bet. You could also check with a local arts council or college art department to see if they know any local artists who work in stone. Also if your community has a yearly arts festival, the organizers might know some stonecutters they could recommend. You might also try phoning some local funeral homes to see if they can recommend anyone.

I'd suggest taking your time to make sure you find someone good. Temporary grave markers are available to mark graves until a permanent stone is ready.