Bought a discovery last week with 21" rims and there isn't much selection for 21" tires for snow and trails.

Found five 20" 72354 defender rims with the wranglers still on them locally. I'm unsure if they will fit properly on the discovery - I am told they will but will they effect the ride with a different offset?

I am new to all of this

 

All 20" Defender wheels will fit a Discovery, no problem. A different offset will not affect the ride quality. Rim size and tire compound will, though.

FYI, the Goodyear Wrangler Adventures are not the best. I had them on my Defender and they're basically a slightly more aggressive all-season. Not that great off-road. Swapped them out for Falken Wildpeak AT3Ws that are MUCH better off-road.