From what I've seen over the years most of the figure focus collections that are started by people "just because" tend to end up being sold off just as quickly as they are put together. While it's not essential, I definitely think the key to being able to continually collect items for a focus collection over a period of years (as opposed to mere months) is to actually have some kind of attachment to the figure.
Childhood favourite, sole survivor of your childhood collection or just a figure you could never get back in the day - whatever the reason, it will help immensely if that figure has some meaning, rather than just being one you think is cool (what if your opinion changes?) or because it's not been done before.
Ultimately if a figure focus it's just something fun you've thought of and have no intention of doing it for more than a few months then ignore all of that
I really agree with Joe on this.
A focus is not something you can expect to complete soon - and nor should it be. Part of the fun (and disappointment), in my eyes, is the thrill of the hunt and finding stuff. I have been after some items for Y-E-A-R-S before finally finding an example. You sometimes can't be too worried about condition too. There are many things I am still looking for.
One thing about a focus is that you find out quite a bit about figures from other countries. I very nearly started a focus just on getting one figure from different countries before the Vader obsession took hold. And its been an obsession for 15 years and still going. I know I won't get everything I'm looking for, but it doesn't stop me from trying
If its just a whim, I'd not worry about it at all. I think there is a lot of scope for Madine (I didn't even think about Lili Ledy above) and it could be very fulfilling if you decided to focus on pre-prod stuff as well. If you're after a cheap focus, though, I'm not sure there is such a thing.
Good luck, no matter which way you go.