I was in the same boat a couple of months ago. New job and winter meant less riding so after many years somehow not doing it decided to try night riding.
Quickly got sucked into finding the most powerful light possible (on a budget of course) and spending hours trawling the web.
In the end I knew lighter nights were on the way so thought best to go easy on the cash and get something to test first that was fairly cheap.
So got myself an ultrafire 501b from torchy the battery boy (big_f_d_d on ebay) along with decent charger and batteries. He threw in a free helmet mount, it’s actually for a rifle barrel but does job.
Works a treat, super bright, lasts a while. Only problem was over bumps it scrolled through modes but putting a post it note round the battery stopped that.
So for about £40 I was sorted and night riding just adds to the whole mtb experience no end. May get a handlebar mounted light next winter but to get started this set up was fine.
Hope that helps.