Surprised to see such prices in UK for used WR250R. As far as fuel range and small standard fuel tank ... AKAIK, most every dual sport bike out there requires a larger tank, especially KTM/Husky. Every bike needs upgrades ... whether a Chinese 250cc or $25K BMW GS. Setting a bike up for travel takes time, money and new parts!

I've modified every bike I've ever owned ... as most here do.

But with 60 to 70 MPG, the WR250R goes far even with tiny standard 2.0 gal. tank. Many reports I've read (US/Canadian owners mostly) do well with stock tank, some returning 70 MPG. Others carry a Roto-pax tank or one gallon jug or bladder.

In any case, buying a larger fuel tank for the WR does not break the bank.
A couple hundred USD and you are there. Or ... you might even find a nice used one

IMS (USA company) make two tanks for the WR250R:
IMS 3.0 US gallon ($247)
IMS 4.7 US gallon ($383)

The IMS is the most popular upgrade but also Safari make a tank. Safari are good but expensive here in USA.
Safari 3.7 US gallon ($574)

In grand scheme, seems the WR250R needs fewer "upgrades" than most other bikes out there. Many retain stock seat (hard to believe but true), many also stay with stock suspension. (I would not) The hard core WR owners do everything as they are pushing the bike into Pro Enduro type conditions.

IMO, this is great for us mere mortals to learn from (or old guys )
Proves how good the bike actually is in tough conditions over time.

Elec. power output is fairly strong for a 250cc bike, so a good thing if powering heated gear/accessories.

Used WR's in USA are fairly high priced. From about $4000 up to $6000 for nice, low mile examples. They rarely drop below $3500 and tend to be well used. Good for long term, but means price of admission is high.
Worth it? I think so in this case ... as the bike is discontinued by Yamaha!