Buying off-plan is certainly not unusual and often even allows pre-construction conditions. Apart from that; the first choices are yours, so you could find yourself in a position whereby you can select the nicest properties available on the project. It is advised, before you sign-up a contract at the pre-construction phase, to ask for a copy of the developer’s land title deed (Chanote) whereby it clearly shows that the project’s land parcel is owned by the developer or one of his nominees.
It rarely happens that pre-sales started before the land purchase is completed by the developer: In such case you better turn around and walk away.
It is also good to ask a copy of the building license; in theory the construction may only start if the licenses are issued. If you sign a contract, please make sure that any deposits are fully refundable in case the project will be canceled or seriously delayed (starting date as well as completion dates).
Buying property in Thailand is often a pleasant experience if you deal with professionals.