Thanks everyone for your input. I'm leaning towards buying 2 bikes. Time and money will tell if I replace one with the other or try and keep both.

Bike 1) a cheap enduro that I can ride on sand and drop as many times as necessary until I learn. Also needs to have reasonable maintenance schedules, not competition-bike maintenance schedules.
Bke 2) a long trip / adventure bike, either replacing or in addition to the first bike, depending on many factors.

Bike 1) CRF230F or TTR230.
Bike 2) KLR / XT600 / XT660

I think the best overall compromise for what I'm looking (sand / highway) is bike 2, but I don't feel confident (given their weight and seat height) with then being the first bike I ever ride on soft sand. So, I'll take the cheap bike, learn the ropes, drop it as much as necessary and then get the heavier / more powerful bike that can also cruise on highways.

Additional thoughts and replies.

On KTMs: I love KTMs, both their design and performance. As I said, I once took the KTM MX school in Devon, and it was on 4-stroke 200EXC. I opted for the one they had set up with 19 / 16in wheels, since I'm short and felt more confident that way. It felt like the ideal enduro bike for a guy with my limited skill and height, although I'm not crazy about their maintenance needs.

Having said that, I can't consider the KTM models proposed because:

1) They are all way too tall for me (I'm 5'6"). I can lean on one side and touch with my toes on something like an XT or a KLR. Anything taller, and I can't really own such a bike.

2) There aren't many older used ones around, and the newer ones are both expensive to buy and expensive to import in to Mozambique. I'll end paying something like 15 - 20 kUSD to bring a 640 Adv here.

XR 250: I'm leaning towards a CRF as poor man's XR, since I'm not finding any recent XRs. My options are:

a 10 to 15 year old Japan import XLR250 Baja / XR250L with unknown miles and history for 3k USD + import duties.

or

- a 1 to 3 year old CRF230F with a full service history for the same price (less than 3K + duties).

I'm quite confident I can make the CRF street legal, too.