You're 18s with all seasons and xdrive should get you around fine in all but the worst weather.
Is the set of wheel/tires he's looking to sell you cheaper then a new set of runflats, as essentially your purchase is now hinged on buying a second set of tires. How long is your lease, from the sound of others, you probably will only get ~30-35k out of your stock tires so you'll probably need to replace before lease turn-in. Do some searching on your own to confirm though.
If you decide this combo is cheaper then buying new runflats at lease end (or you could swap to non-runflats now and keep your runflats for the turn-in) then you can essentially double the life of your runflats by swapping to the winter set for half the year and get the added benefits of better winter driving and have a set of wheels to sell at the end of the lease.
Just trying to put it into a cost benefit analysis for you.
Herbert - '12 X5 35d / Sapphire Black / Black Perf Nevada / Aluminum trim / ZPP / ZPS / ZCW / ZAV / Apps / 3rd row
Betty - '04 CTS-V / Black Raven / Black Leather & Suede / 6 speed LS6 combo 400 hp & 400 tq
Sterling - '06 Silverado HD / White / Grey Leather / LBZ Duramax - Allison 6 speed combo / Crew Cab Short Bed LT3