You could check how much space your photos and videos are taking up on your current device. That will show you roughly how much space you would have used on an S6 (depending on resolution/quality settings).
I'd probably buy another S5 for a lower price. Batt, SD, water/sand resistance, USB 3 and Kies rather than that reduced feature Smart Switch thing. I'd even consider a Xiaomi Note 4 Pro.
When you import for other people, that is distribution not personal use.
Duty might be calculated on the value of the item. I don't think 'gift' will work for phones. Check with customs. They'll know for sure.
The difference is not being in debt, and possibly missing a payment, and not having to remember when the money leaves your account. Nobody knows how secure a job really is, and sometimes things do get forgotten or misread.