My single person three season tent is the Big Agnes Copper Spur UL1. It is very much not a "stand up in" tent (troischiens), but it's very lightweight. I've never used it in severe weather, either heavy rain or winds, but I've been happy with it so far. I plan to use it in the winter, too, but never have. My winter camping plans would be cancelled if high winds or heavy snow were forecast. I packed it in eight miles each way last fall for a two night trip that went really well.
I recently bought an MSR liquid fuel stove. I think it's the Whisperlite? It's supposed to be good for cold temperatures. The big purchase I need is a winter sleeping bag. I've sort of settled on the Big Agnes Blackburn UL but am waiting for a sale or coupon.
I also might need a bigger backpack. My 65 L pack gets pretty full even in the summer, though skipping a bear canister would gain some space. More and bigger gear probably means I need an 85 L bag. I'm looking at this Osprey one.