I have been in this dilemma for a long time now, and feel your pain. MOst have seen my "Skeeter" project gun I have been working on for 20 some years so I wont repost it, but it is very special to me and I believe it needs a bit of finery.

Here's some thoughts
Carrying and using one you have engraved will not affect the value as it would if it was factory.

Anything can be engrave even nickel, but you will incur the expense of having the nickel removed prior to engraving.

Get with someone who is a master of the art and through conversation they can usually help you come up with a theme.

Lastly, it is an expensive and time consuming process so make sure the gun you choose os worthy whether sentimental or whatever.