Oh nice, know a few people who have done that

. Think Bob from here has and a couple of my Dutch mates too. I’ve heard that studded tyres are worthy investment for the trip, a couple of years they had a freeze thaw and studs were required for large sections.
As ever with winter fat bike races, get used to pushing (and falling over in deep snow!)
Pogies will be a good investment and I found several pairs of those insulated builders gloves to be perfect (several pairs as they do get sweaty!).
Clothing wise, vinted as suggested above is excellent - think eBay for clothes but with set prices, so it makes it easier to shop.
I was in Jack wolkskin on Cheshire oaks last week and they had down jackets in there for £60, might be worth a call. Jack wolfskin is also very generous on sizing too
Weird son on a bike or weirdos in a bike (can never remember which is correct

) had some down pants on their website when I last looked, seemed reasonably priced too. Saying that, I only put on primaloft 3/4 pants whilst doing the Rovaniemi races once and that was in the start line of one race and one early morning on another race when it went to approx -20C/25C on was descending.
Definitely grow a beard, it’s in the polar rules of expeditions…!
Don’t forget over Manchester/Derbyshire way Mountain Equipment and Rab have their own outlet stores, can’t remember their names but sure google will tell you.
I known someone to have waiters until they got to the resort for the race (this was Rovaniemi and not fat Viking) and bought a load of XC skiing clothing and they swore by it but a bit of a risk if you don’t find anything.
Finally, check sportspursuit and sports shoes as they’ll have plenty of discount clothing too