The Trek 850 wasn't exactly low end. Many of them had Double Butted Chromo frames. They were better than the 800(sometimes just seat tube cromo) and the 820 830 (usually full cromo but maybe not the fork).The 900 series had better True Temper OX DB tubing- with better- but not great- components.
Still since you can buy a good one for under $150- frequently under $100- kinda pointless spending and $$ on them.
OK frame, but not worth spending any $$ on.None of them are worth converting to disc.
You can get a used 853 -disc- frame off ebay(Jamis Dragon) for about $200.
Good frame - cheapo components= good bike.
OK frame- great components= OK bike.
Cheapo frame- world's greatest components= cheapo bike
Don't bother- you have an ok not bad not great frame.
Charlie