Typically condo buildings have available up to 49% of the space reserved for foreigners and 51% reserved for Thai ownership. If the quota is not taken up entirely then a Thai owned unit can be sold to a foreigner and it will become foreign owned, adding to the quota accordingly. If the foreign ownership quota is full then the following is allowed. A foreign owned unit can be sold to another foreigner without affecting the overall balance, since nothing is happening to change the status quo. However if a Thai owner wishes to sell to a foreigner he or she will need to sell it with company ownership – essentially this will create a shell Thai owner with the foreigner as director of the company. This is a very much less preferred method, as it will entail providing company accounts each year and other requirements. It will also mean that it is not a free hold foreign owned property. 
So it is best to check that the unit is to be sold by a foreigner and is already a foreign owned unit. In the case of dealing with a
developer where units may not yet be allocated it is important to seek a guarantee that the unit will be available for foreign ownership
as often this is left until a last minute rush at the land office where a lottery can ensue. Next is to check the location of the property. To put you in good stead it is worth ensuring that there is good access to local shops and if possible to the city center. If the property is near a beach then this is also a benefit especially as a selling point at the time of resale. Of course you will also retain the ability to enjoy the beach yourself too! 
Build quality is next on the list and it is important to ensure that the place is well built. If there is any doubt then a look at other projects that the same developers has had involvement with will bode well. However slightly deeper research may be more beneficial and to this end it is worth checking which building contractor was used on previous projects and matching it to the building contractor used on the development you are interested in. That way you can be rest assured build quality is up to scratch.
In a similar way to build quality, maintenance of the building will be all too important. A condo can deteriorate very quickly if there is poor maintenance in play and so again it is worth checking how the maintenance for previous projects of the same developer faired. If this in hand then you can be reasonably sure that your investment will be sound. 
Fixtures and fittings are next and whereas in the past shell properties were common, there is a drive towards selling fully fitted and furnished apartments. This in part justifies the higher prices of late, but it is important to check if the unit you will buy will have exactly the same fittings and fixtures as the show room that is presented to you. If buying from the resale market it is important to check that all of the furnishings and fixtures seen at viewing are included in the sale and to make a list thereof. Furnishings can be expensive and so even a small omission will be extra cost for you.
If you want a quiet life than choosing a condo unit that is away from a highway will bode well. Also it is worth choosing a condo that is not located near to a lift as people can congregate in this area waiting for the lift to arrive. As for noise from neighbors, then choosing a corner unit will be sensible, as it will avoid noise from neighboring units. The build quality of the entire project will also dictate if you will hear noise from floors above, but you could choose a top floor apartment to ensure that no one is walking about above you. Be aware though that top floor apartments can sometimes attract the heat of the sun. Still, moving higher will also move you away from traffic noise and any other noise at the base of the building. With noise in mind be sure to fit  a solid front door to the apartment if this is not already done.
Many buyers deem a structural integrity check unnecessary, but in an older building it may be worth hiring a surveyor for peace of mind regarding a buildings structural standing. Mot condos are built very strongly so this is usually a formality. In Europe and the US where buildings can be much older then a structural check is common since there can be settlement of the building over many years.
Another important check is to ensure that electricity and water are provided on an uninterrupted basis and that there is no risk of flooding in the wet season.
Finally is price. Be content that the price you are paying reflects the market price for that location, that developer and that size and standard of condo unit. If the price is too high then it is worth negotiating. This can even be done with a developer where free add-ons may be able to be secured. In the resale market then there is usually room for bargaining. Be sure to include in your budgets the cost of stamp duty, business taxes, lawyers fees, escrow costs, estate agents fees where necessary (though the seller is normally the one who pays these) and the cost of any renovations or alterations to the property once you have completed the purchase. Only then can the true price be know. 
With the above in check you should be able to enjoy peace of mind that the condo that you have just bought is all fine without worrying about leaving anything to chance.