also, I'd just like to point out,.in case my parents are reading this, I have no idea what I'm referencing but I probably learnt it watching Grange Hill and they were right all along
Just book a hotel on Agoda for a couple of nights, wander around and see if you find somewhere that suits you better and book there, or several. Few places are at capacity currently and you won't have an issue finding accomodation. It's not a big city. You may prefer old city, you may prefer just outside. Work it out when you get there
Dtac, AIS and try are all good in Phuket. Dtac probably have the cheapest and easiest prepaid plans for your needs. Avoid the "tourist" Sims at the airport if you dont plan to call home and just need local and data