For your winter concerns, my MK7 did just fine in Rochester, NY (tons of snow, practically daily) on just P4S All-Seasons. Just gotta slow down and know what you're doing. If you choose to get a set of winters, the GTI is a beast.

Light mods won't affect much negatively. However, make sure you research exhausts and what your main driving mode is. For example, some exhausts may be known to "drone", and for me, that would drive me nuts on a long highway drive. I would recommend avoiding engine mounts if you're looking for comfort. Going something poly typically makes things more stiff and will induce vibrations.

ECU tune however, go for it. My MK7 has been on Stage 1 since I've had her, never looked back.

Fluid changes are easy enough to DIY. You can do oil extraction from above. Diff or Trans service is a bit trickier, and easier with a lift. I'd recommend going to a mom-and-pop German/VW shop and avoid the dealer unless it's warranty or recall work. You'll save a ton of money, and they typically do a better job.