Hi there, we are off to Tenerife next Friday to the Anthelia next week and can't wait.
Are you away for a week or a fortnight? If its a week would you not be better just bringing the milk powder with you and one of those mini dispensers for mixing up bottles. You can get sterilising microwave bags or if you don't have a microwave then bring the tablets.
Definitely bring swim nappies, they will be about triple the price at home.
Babies abroad need nappies too, so if yo uneed to buy any you will be fine!
Car hire is not too expensive, most car seats from car hire companies wherever you go tend to be a bit dodgy and expensive probably 30-35 euros for a week, so I would see how much it would cost to bring your own on the plane or contact the resort to see if they have any ideas.
To be honest I'm not sure that a nursery abroad in a hotel would get your DS used to going to nursery at home , he may well not like it and kick up a fuss about going, so if you have to pay in advance for it you may wish to consider this. We found evening babysitting worked best at that age, but having said that your DS would probably sleep in his pram/buggy if you take him out at night.
Also re your location, at your DS's age he won't be much interested in the entertainment or swimming possibilities I'm sure your hotel will be lovely.
Not sure about the pool, I'm surprised its not heated at least a bit, again maybe check with the resort, or read reviews on trip advisor.
And brrreaaatheee. You are in Tenerife not Outer Mongolia, they have experience of babies and children, they sell pretty much everything in the shops, have a lovely time. 