Well if you want to be away from the snow, you can rule out Heljarchen Hall in The Pale. My current character lives there and there is always snow on the ground, plus the Pale is allied with the Stormcloaks.
Hjaalmarch and Falkreath both ally themselves to the Imperials, so I don't think there is a lot of difference there. Winstad Manor is perhaps a bit closer to Solitude the Imperial centre of power (and hence Thalmor centre of power) and a large city but Lakeview Manor is near mountains cutting off the nearest Stormcloak allied hold and is in close walking distance of both Falkreath and Riverwood so I've never had trouble selling lots of stuff from there.
I suppose it depends on what sort of character your Thalmor is for picking Winstad Manor or Lakeview Manor. If you assume the Summerset Isles are a sunny warm climate (which I always have but have no Lore to back this up, Head Canon), then I'd say go for Lakeview Manor as it would be closer to the climate of the Summerset Isles than Winstad Manor. However, if your Thalmor is serious about alchemy or perhaps grew up eating lots of fish (Hey, they are islands, so surely they'd be lots of fish on the menu in the Altmer homeland, right? Head canon), then Winstad Manor might be a better bet for the fish hatchery.
For not making the place too grand, I'd consider just building a small house or a main hall and ignoring the wings and cellar. Small house will give you a bed, chest, cookpot, garden and smithing equipment if you build them outside. Building a main hall will get you these as well, plus an enchanting table and alchemy lab. What else do you really need?
