Sarah,
I don't know what the baggage allowance is but from personnel experience, travel light going to Thailand, as you'll end up doing a lot of shopping and then you’ll need to buy another suitcase to bring it all home.
Two weeks in Koh Samui is probably too much, unless you like lazing around. The shopping won't extend much beyond the typical touristy souvenir and fake clothing and watches that's so common in Thailand. You need to spend a few days in Bangkok for the serious shopping.
You do need to haggle a little, it’s built into the prices they ask, it's off-putting for a lot of people. They don’t expect you to pay the first prices they ask for. Trying to knock 10Baht off a 50Baht souvenir is no big deal. They still have to make a living and feed a family, But if you’re spending say a 1000 Euro on handmade clothes, then you have room to bargain hard, for example, better cloth, a couple of free shirts or dresses etc. A 1000 euro each should be more than enough. Thailand is a cheap location once you get there. Do try to take a boat trip or two.
Do try their food as it's great, you'll be missing a great opportunity if you stick to pizza and burgers! The hotel food should be fine and will cater for all tastes.
Enjoy the trip there, get your vaccinations in good time and don’t drink the tap water there and stare alert for the jewel scams
.