Sounds similar to my situation. I recently converted my 26" MTB into my winter commuter on a "budget". The frame/fork could accommodate disc brakes already though so what I bought was
- new set of 26" disc or rim brake compatible wheels, wider rims than existing (26mm instead of 20mm I think)
- rotor + brake for front disc brake (used existing v-brake lever). Avid BB7.
- disc brake cable set (enough for 2, still 1 left)
- new brake pads (for rear v-brake)
- nokian 160 studded tire for rear
- nokian 240 studded tire for front
- tubes for 26 x ~2.0
- shambolic fenders. $5 on ebay and broke already but I jerry rigged them up again. may need to upgrade but i'll try them first
- new 7 spd cassette (13-34) + spacer for cassette. the existing rear wheel had a 7 speed freewheel 14-34 with too much spacing at the low gears imo.
- lights/flashlights (
TBD - I have various options available)
I think fenders, lights if you're riding in the dark, and perhaps studded tires if you're worried about ice would be the most I'd do to that bike if you're going to get another one next year.
My MTB was 30 lbs before I made the changes so it's a heavy bike but I plan to use it as configured only when I need studded tires.
I did swap out the handlebar, stem and seat with my unused hybrid to be a bit less MTB and more road worthy, and lighten it up a touch. I also switched the bar ends from their normal position to upside down, inside the grips, and reversed (R <=> L) so they act a little like drops. I had bar end extenders laying around that work on my MTB brake levers that lets me brake from these "drops". The total cost of my upgrades was close the cost of the bike, new, but the new wheels + Nokians are an investment for years to come and can easily be moved to another 26" bike. And the disc brake + rotor could be re-used on any bike with a disc compatible fork.
I also plan to get a more tailored commuter sometime (700c with wide clearance, drops + disc brakes, wide gearing, not too heavy), perhaps next year, but this will be a good wet/winter/spare bike in the meantime. I still have the slightly lighter old wheels (rim brake only), 26x1.25 slicks and 26x~2.0 knobbies so I can use it for other purposes.