I've been using Palms since March of '96, when I bought my Pilot 5000 the first week they were out -- we currently have Trēo 680s. I know a few bits of information about
Palms in general that look like they might be useful, so I'll toss them out.
Just so you know what I
don't know, however: I've never messed with swapping SIM cards, nor traveled abroad with my phones, so I'm not going to comment on that, other than to suggest that you confirm whether Palm phones might already work abroad, I'm under the impression that they
might.
All Palms have the Palm OS functions you're asking about, completely independent of phone service, or even phone signal. I've never heard of, nor encountered anything about two "modes" so I'm not sure where that's coming from. Web access on a Palm is by way of a web browsing application. You control how that connects to the web.
You might also want to look at the Palm Centros. They weren't out when we got ours or I'd've given them a good, hard looking at. I don't know which will fit your needs best, I just want to make sure you know about that option too.
As for locked versus unlocked, if you're wanting to get phone service anyway, you
might want to consider going with a locked, discounted one. There are a butt-load of online services which will provide unlock codes for phones, including Trēos, for under fifty bucks (just Google "phone unlocking" for a sample). For a 50% discount on the phones, I decided it was worth the hassle of possibly unlocking them at some point in the future.
I don't know the details of your situation, so I haven't the faintest idea whether you'd want to go that route or not. I just wanted to make sure you knew it existed.
Hopefully, that'll help some.