The WR250R has 26,000 mile valve check interval. The WR400F/WR450 are race bikes, so intervals will be less. It's a more highly tuned (more HP) engine. But I guarantee you can go well beyond 500 km. on your WR400F.

Especially traveling, you'll not be racing so you can stretch intervals a lot ... like at least double. Several friends raced WR400's in Super Moto. They got 3 seasons out of a motor, racing nearly every weekend during 4 month race season. No blow ups. Valves hold spec well and oil does not break down ... so 500 km. is very pessimistic, IMHO.

The WR250R comes in a mild state of tune, designed to run fast on highway all day. It's been around since 2008, very few failures. You DO have to change the oil more frequently but can probably go further than the 400F depending on USE and Abuse. (I'd guess every 2500 mile oil change)

You could go with Suzuki DRZ400, bit heavier than WR250R, more power, reliable, easy to maintain ... and cheaper than the WR250R.
BUT ... Old design, Carb not F.I.

No question the WR rides lighter than a DRZ (owned DRZ-E for 3 years) ... and WR250R a better road bike. So smooth!

IMO, only real competition might be the new Husky and KTM 350's. Both over
$10,000 USD. You can find a nice, low mile used WR250R (In USA) between $4500 and $6000 USD.

Since the WR has been around nearly 10 years it's well supported in the aftermarket ... and LOTS has been learned about doing mods from owners.
Lots of new, better products coming out for the WR all the time. So Much Too Learn about this very popular machine.