1) I would book my flight 2-3 months before the departure date since the prices seem to increase one month or so before (talking about Orbitz or Expedia)
2) I would wait about a month before booking a hotel and I would book it with Agoda. They seem to have the most updated prices and best discounts. If you book a hotel now, the prices may be higher than if you wait until slow season. The hotels in the Silom area that are decent and reasonably priced are: Rose Hotel; Rooms@Silom Hotel; Tawana Bangkok Hotel; Unico Grand; Rose Hotel; Bally Studio; Pinnacle Lupinee Hotel, I Residence and Silom Regent ( a bit small but good location). I have stayed at all of these the last few years and their prices are under 1600 baht- check Agoda for prices (peak season now but reduce by May). Some of the hotels have small pools, others do not. But they are all walking distance to soi 4 and soi 2 (unless you are unhealthy ) I agree, stay away from the international chains...too many families and they do have visitor charges.