Honestly, I can only think of a KTM 640 Adv, tough bike, takes loads with panniers, decent long range (350-400 km at that speed), comfortable, surprisingly agile on sand, gravel roads and tarmac, parts available in SA, you can possibly find one with low km on the clock, as recent as 2006. I'd suggest to find one anywhere in the world and get it shipped to you. Ride safe.

KLR 650, a little bit ov a workout on single track trails, but cruse on the tar at 85mph, all day long

I have gone places on my klr that some would not dare on a 125 motocross

Highly recommend as a great all rounder

XT660Z ticks all those boxes.

I'd stay away from the BMW 650 singles - unadulterated shite IMHO.

KTM 690 Enduro R would also be high on my list.

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.

I think most 600-700cc single cylinder dual-sport motorcycles would fit what you're looking for.

Soft sand is very different from most other types of offroad terrain.
Honestly the best bike for soft sand is a 2 stroke 500cc Motocross bike with a paddle tyre on the rear.
Lightweight for the HP and the paddle tyres make a big difference, soft sand is funny stuff, once you chop the throttle the front digs in and the whole bike changes attitude.
Is the sand offroad = unregistered bikes? or is the sand part of roads that just happen to be in sandy areas and the bike still needs to be street legal?
The more power you have in sand the easier it is too get on top and float along.
250cc 2 strokes will still perform OK, but big bore is better so think about 300cc+ 2t Machines if you want to have fun and bring the bike in the truck.
Forget about big twins in sand, they are exhausting. They have the power but also the weight and are a handful when you drop them over the course of a days riding.

But the KTM has a 37" seat height (OP is 5'6" ), great off road but will vibrate your fillings out on highway. Worst vibes of any bike I've ever owned.

I think you are on the right track with 250's. I've ridden the CRF230 a lot and prepped one for off road for a friend ... so I know the bike well, have ridden it lots and know it's potential. The CRF230 is (I believe) one of the Brazilian made bikes. They are quite good, reliable. Don't shy away from the Brazilian bikes.
ALL GOOD.

The CRF230 will need to have the rear shock re-done and heavier springs up front. Carb will need a minor re-jet and less restrictive air filter. Do that and it is transformed to a really good trail bike. I'm 5'6" too, and have struggled with TALL dirt bikes my whole riding career. No fun.

If you have the budget you might also want to look at the new Honda CRF250L. This Thai made dual sport bike is really doing well and many are doing RTW on it.

Also look at both the Yam WR250R and KLX250S. The WR is Yam's top of the line in the 250 class, but expensive and rather complex. The KLX is cheap and cheerful but needs modifications to match the Honda (CRF250L).

None of the 250's will be "fast" on highway, save the WR250 Yamaha. But ALL will cruise OK at 50 to 55 MPH. But off road in mud and deep sand, the 250's will be a JOY compared to any 600cc class bike or even the older 250's like the XR250L, KLR250 or DR200. The 600's are way too tall, heavy for mud and sand. The old 250's are OK, but if you can
afford something better, do it. The CRF230 is an EXCELLENT choice, IMHO!