The French soldier of WW1 gets a very bad rap these days and was, in reality, the true hero among the allies of WW1. This is not opinion - it is well substantiated fact which we unfortunately tend to forget in our rush to glorify e verything English and depreciate everything French.
To get a little off topic. I agree; but also for WW2 as well. When the Germans were invading in 1940, in one month over 100,000 french soldiers were killed or wounded. That is a higher casualty rate then any one month period during WW1.
To get back on topic. My opinion is to get something that you are interested in, not just something to maybe make money in the future. And when I say interested I mean the whole 9 yards. Something that you will get mountains of reference material, and spend days/weeks just learning all that you can about that particular type of rifle. I've started doing this in the last year, with Enfields (look at my signature)
For instance, look at
this New Zealander. He obviously is a hard core SMLE collector. He has quite a few pieces that are rare (the MkV and the full length skeleton rifle).
Personaly, I also want to pick up a 96 or 38 Swedish mauser, and an SVT40. Not to make money, but to have as part of the collection and to shoot.
EDIT: Also work on getting all of the bits an pieces that go with the rifle. Bayonets, frogs, stripper/charger clips, slings, mag pouches/bandoliers or whole kits, armourers tools, and so on. So that if you are just in it for invenstment, when the time comes to sell, you have a complete package.