Dealer recommends changing out brake fluid and fuel treatment. Necessary? 51 k

NO... Hell NO!! .. Is the brake fluid in the master cylinder still clear? .. They're trying to sell you those services... Weather the car needs it or not! .. I've been at this game WAY too long for that BS... Other opinions will be posted and welcomed..but you know what they say about opinions... 43 years in the business, so my answer isn't based solely on just an opinion!!! .. 'IF' the fluids are obviously in need of being changed, then I do so! .. Don't waste your money.

there are test kit to test fluids for acidity or deterioration, regular interval maintenance is always a good choice most manufacturers recommend fluid replacement every 30k is cheaper to service than is to repair

At age 74, I agree with pushrod. The only reason to change the fluid should be its condition, not an odometer reading. The only thing that could go wrong with brake fluid would be if it absorbed water, in which case it would look nasty.